IF CARL MARIA vonWeber were in a position to sue, he would have an excellent case against the Maximilian Worldwide Lyric Opera Company, which is now playing in the Kennedy Center's Terrace Theater.

Or rather, since the company is fictitious, he would have a case against Hugh Wheeler, the Demon Barber of Broadway.

But Weber died in 1826, and his work is defenseless against Wheeler - who, aided and abetted by the Washington Opera, has introduced enormous distortions into Weber's lightweight masterpiece, "Abu Hassan."

Wheeler's hands are still bloody from working on the Broadway script of Stephen Sondheim's "Sweeney Todd: The Deomon Barber of Fleet Street." And now for the Kennedy Center, he has taken not one but two classics (the other is Mozart's "The Impresario"), hopelessly mixed them up, and daubed them liberally with the dayglo glitter of Broadway.

This elegant piece of stitchery (which happened partly because the Kennedy Center's artistic director, Martin Feinstein, enjoyed "Abu Hassan" in his tenns and wanted to see it done again) may launch playwright Wheeler on a new tangent from his career, which has centered until now on Broadway, Hollywood and mystery novels. He has worked on quite a few musicals (Bernstein's "Candide," Sondheim's "A Little Night Music," the screenplay of "Cabaret"), but this is his first venture onto the operatic stage and it will certainly not he his last.

On a brief visit to Washington, before going out to Hollywood to see a roughcut of his new film on Nijinsky, Wheeler hinted that he may take a look at some old Viennese operettas, which have music that is still alive while the scripts have become out-of-date. Reworking old material in this way has been a part of show business since Shakespeare (or since Euripides, for that matter), and Wheeler is all for it: "Having invented 30 detective story plots, I am now very anti-plot; I want someone else to do that. We playwrights are traditionally very lazy - why invent when you can steal? At the same time, you get a proprietary felling about the material. I don't think of it as Weber's 'Abu Hassan' any more; it's my 'Abu Hassan.'"

Whether it's Weber's or Wheeler's, there has never been a performance of "Abu Hassan" quite like the one now at the Kennedy Center. After exposure to Wheeler's razor, the opera has backstage problems coming out of its ears - and into those of the audience.

What Wheeler has done is quite unprecedented; he has taken two completely independent works, written by different composers in different centuries, and made them one. He has made the soprano-rivalry theme from Mozart a frame for the Weber, an introduction and a whole world of performing arts.

His fictional contrivances go like this:

Joanna Brinkman, the prima donna, is in love with the show's assistant producer, who has used a lot of chicanery and manipulation to get her the starring role.

Two other sopranos who wanted the prima-donna role instead have been shunted off into bit parts. Enraged and jealous, they are sabotaging the production by hogging the spotlight whenever they get on stage, wooing the audience shamelessly, breaking into song although they have been given speaking roles and the orchestra refuses to play along.

The bass-baritone, Emmanuel Schrimpen of Stuttgart, has a lot of problems with English - the language in which this production is presented. He would rather be doing Wagner, and has made this perfectly clear.

The company is deeply in debt and in imminent danger of having the theater taken away from them and turned over instead to a group of performing poodles. Perhaps these financial problems are one reason why they have chosen to do "Abu Hassan," which is also about people desperately in debt.

Soprano Brinkman is singing her first role, and everybody in the company (except the other sopranos) is worried for her and rooting hard. They crowd in the wings backstage when she is singing, and their efforts to encourage her can often be seen by the audience. Her lover is particularly offensive in this respect. Sometimes he goes out into the audience, leaning over the edge of the orchestra pit and gazing at her, moonstruck. He has even been known to walk onstage during the performance, carried away by his enthusiasm, or to reach out tnd hold her hand from backstage while she is singing.

Wheeler's framework is terribly distracting, and it transforms the performance - turning a pleasant but rather contrived little comedy into a farce. People keep laughing at the wrong places, and they seem to be more interested in the newly-added backstage problems than in "Abu Hassan."

In the case of Mozart, Wheeler's offense is less flagrant. "The Impresario" was written as incidental music to a long-forgotten play, and for all practical purposes the music is all that remains. Wheeler has left the music untouched (except to add another neglected Mozart aria to the score) and simply written a new play around it, laying out in very comic detail the temperamental and financial problems of the Maximilian Company, which will then perform "Abu Hassan" as the second part of a Siamese-twin double feature. One who holds the classics in a certain hushed awe may despise him for this, as though he had painted a moustache on the Mona Lisa. Others will bt too busy laughing.

Wheeler's next project is also operatic: He is busy performing rescue operations on a Kurt Weill opera unperofrmed since its premiere when it was suppressed by the Nazis. "Harold Prince is going to direct it for the New York City Opera," he said. "The libretto is a German expressionist work - more or less incomprehensible - but it has a thread of a story, which I have pulled out."

If he stuck to writing serious plays without music, Wheeler conjectured, he might have box-office problems on today's Broadway. "I really wouldn't know what to write. I like to do quiet plays, which don't work anymore. People want special effects - 'Dracula,' for example. You give them bats flying around and a coffin, and they think they have their money's worth."

Writing the "book" for a musical show is a very different experience from writing a straight play, he added. "For one thing, the success or failure of the show depends on a lot more people, and of course you don't get as much credit. Personally, I love working with Steve Sondheim and Hal Prince, and the fact that the bulk of the credit for a success goes deservedly to them doesn't bother me at all. I'm not very competitive; I write for the pleasure of writing, not in the hope that a maitre d' will recognize me."

Writing a new script for "The Impresario" involved a lot more original work than "Sweeney Todd," for example, which is based on a new play based in turn on an old tradition about a "demon barber" who murdered his clients. "We all felt that 'Abu Hassan' needed a bit of help for modern performance," Wheeler said. "Audiences expect so much more. If you can't give them cars going over a cliff, you have to give them something else. As it turned out, the performance of 'Abu Hassan' can be considered a love scene - a bad love scene - between the soprano and the junior impresario who got her the role."

It is also, its new form, a work of wheels-within-wheels intricacy rare in opera - which tends to be dramatically more simple-minded than straight plays. Only two operas of the past come to mind which resemble, even slightly, Hugh Wheeler's amalgam of Mozart and Weber. A fairly simple one is Leoncavallo's "I Pagliacci," which has a play-with-in-a-play echoing and distorting the problems of the dramatic situation in the opera itself. There is considerably more complexity in the Strauss-Hoffmannsthal "Ariadne auf Naxos," where, as in the Wheeler opus, there is a combination of spoken and sung roles and the dramatic situation laid out in the first part is reflected in the way the second part is sung.

In "Ariadne," due to circumstances beyond their control, a serious opera company and a commedia dell'arte troupe are forced to perform on the same stage simultaneously - Punch and Judy, as it were, blundering into an episode from Greek mythology. The two sharply contrasted styles of art interact with an explosive comic effect, but they also blend to create an intricate, hybrid new form - too unstable to last for more than a moment, but fascinating while that moment endures.

Wheeler's production is purely comic, without "Ariadne's" delicate little touches of the tragic vision, and his focus is on the vagaries of opera as a performing art, but he achieves some of the same kind of intricacy, he exploits cleverly the tension between "real life" and play-acting, and he makes the contrasted elements that went into his show work together for maximum effect.

The result is a sort of "Ariadne auf Broadway." And when other opera companies become aware of what is happening this month at the Kennedy Center, it may also become a bright new addition to our country's permanent operatic repertoire. CAPTION: Picture, Hugh Wheeler, the Demon Barber of Broadway. By Margaret Thomas - The Washington Post