Public television may be very close to reaching the outer limits of stooping when it comes to raising cash. As part of the garage sale it's now holding, public TV has featured the kinds of programming only trotted out at festival time, and yet grinning hosts and hostesses keep insisting it's the regular bill of fare on public TV.

Are the fund-raising weeks aimed strictly at the people who only watch public TV during the fund-raising weeks?

We must be tolerant, because they do need the money, and the cloying, jabbering appeals do seem to produce it. But only the very brave will want to sit through all of tonight's shivaree "Broadway Plays Washington," a loud but lackluster evening of show tunes taped in the Kennedy Center's antiseptic and cavernous Concert Hall, airing at 8 on Channel 26.

The three-hour program will be interrupted four times for the usual impertinent importunings, but this is one show that may make the interruptions look good.

All-star extravaganzas are suddenly the thing in television; first, last week's "Night of 100 Stars" and later this month, Norman Lear's bombastic "I Love Liberty" from liberty-loving Los Angeles. The PBS Broadway potpourri is big on celebrity but painfully short on imagination. Tony Stevens, who did the staging, was basically the maestro of a long, dull parade. Stan Lathan, who directed it for TV, just jumps around at random from shot to shot, with no sign of flair or heart.

It amounts to an orgy of hamminess as the actors play very broadly to the audience in the hall and thus look ridiculous on camera. The program has about it the rather desperate sadness of those big-band nostalgia concerts PBS is always showing, and it also has the shouting hype one would like to think would be limited to commercial and pay TV. Shouldn't public TV be a haven from something?

Pearl Bailey does a couple of "Hello, Dolly!" numbers, Imogene Coca romps through "Repent" from "On the Twentieth Century," Chita Rivera does the "America" dance from "West Side Story," and Robert Morse reprises, though far too busily, "I Believe in You" from "How to Succeed in Business Without Really Trying."

And so on, one after the other, with a sequence on show-stoppers by those who first stopped them (John Raitt does "Soliloquy" from "Carousel" again) and another segment on young talent. Melba Moore applies her hideous horror-movie shrieks to two songs--her helpless victims--and Debbie Reynolds, the emcee, continues her campaign to smother all hope in aggravated cuteness.

An announcer bellows out introductions just like on network TV ("Ladies and gentlemen, the romantic Mister Barry Bostwick!"), and one of the underwriters is Shell Companies Foundation, as part of its support of "the arts, education and other charities." The arts, a charity? There's an awful lot of slack thought at work on this production; the show has no real point except to raise money for PBS so that it can put on shows that, one hopes, will be absolutely nothing like this one.