Buyers at the latest round of fall fashion showings here who looked beyond the headgear and hemlines got showings of a different kind: Many designers had filled their front rows with celebrities. . . At Halston, it was Bianca Jagger (with a hole in the knee of her black pants), with Andy Warhol and Steve Rubell, head honcho at disco Studio 54; at Calvin Klein, Ali McGraw and ex-husband Bob Evans, the head of paramount pictures; Diane Keaton was at ralph Lauren; and at Eleanor Brenner again, Joan Mondale.

In Europe one often sees a cluster of designers in the back rows of each other's shows, a pattern that seems to be taking hold in New York as well. Chester Weinberg was at the Calvin Klein show; John Anthony at Kasper's; and Milanese designers Gianni Versace and Carlo Tivoli showed up at the Stephen Burrows show. Versace scored with Karl Lagerfeld, Geoffrey Beene, Diana Vreeland and C.Z. Guest at his own show, which presented all his designs for Genny, Callaghan, Complice as well as Gianni Versace, his leather line.

Though store presidents get front-row celebrity treatment at the shows, it often appears their minds are back at the store. Between shows, many head to Bill's on W. 40th Street for steak sandwiches and creamed spinach. Spotted snacking at the Flair, the coffee shop next to 550 Seventh Ave. (the building that houses the biggies' showrooms) was Neiman-Marcus president Philip Miller. Val Cook of Saks-/Jandel heads to Flair for a coffee boost between shows, always sitting up front where waitress LeVerne holds forth.

Hands-down winners from New York's fall '78 designer collections: Mary McFadden (now designing for day as well as evening), Geoffrey Beene, Bill Kaiserman, Stephen Burrows, Perry Ellis and Calvin Klein.

Essential accessories from the New York shows: cummerbund or wrapa-round belts (and cummerbunds that wrap around); high-heeled shoes or flat, mannish shoes; ankle high boots and high wedge boots; long bouncy hair or pinned up hair (no Brillo-like hair); long, dangling earrings; funny hats and caps.

Buyers used to haunt Seventh Avenue fashion shows to find out how designer thought their clothe should be accessorized - what hat, heels, hose, etc. Now it's all become such showmanship that one must skim away the extras to try to see the clothes. At Bill Kaiserman for example, a group of models appeared, laden with accessories and, of all things, stroking (stuffed) falcons. Only after all the extras were pared away could you see that Kaiserman's may well be the best clothes in New York.

Karl Lagerfeld, another designer with a great sense of showmanship, is now saying that wasn't a prison that he set up at one end of his runway for his Paris showing show earlier this month. "I had been walking about Paris and had seenall the beautiful gardens and gates," he said later. (But how does he explain the model with a huge ring of keys dressed like a "jailkeeper" at the start of the show?)

Pitched on the cover of the current Harper's Bazaar, a story "Can Hypnosis Enlarge Your Breasts?" By willing their breasts to grow, according to the story, 22 women volunteers in a study by Richard Willard, M.D., at Cameron Memorial Hospital in Indiana gained an average of two inches in their bust measurement -maximum was four inches. What next?

What does an 18th century-minded Paris gentlemen designer want most to do in Washington? Karl Lagerfeld, his director Gaby Aghion, pal and party organizer Jacques de Bacher and Paulette Dufault of Elizabeth Arden headed straight to the Smithsonian's Air and Space Museum when in town last week. Offered a swanky lunch by Garfinckel's Aniko Weiner who was with them, Lagerfeld said "non," insisting that the group have not dogs and cokes from a vendor - and then picnicked on the front steps of the museum.

Among the Washington models doing the New York shows: Treena Brown at Holly Harp, Carolyn Korneman at Rafael, Gerri Maxwell and Charlotte Fleming at Willi Smith, Jennifer Brice at Stephen Burrows, Charissa Craig at Mary McFadden and Gloria Burges at several of the shows.

You just can't get Opium. Yves Saint Laurent's latest fragrance is almost as hard to get as its druggy namesake. At the moment it is on sale only in a few cities in Europe. Nor problem here because of the name, says Marina Schiano, only the short supply. That didn't stop Sally Marx who with husband Ernest Marx bought up all the Opium (the smelling kind) in Paris and brought it back for their Rive Gauche boutique at the Watergate. They've priced it at what is expected to be the price when it's officially on sale - $115 for the ounce, $90 for the half ounce (true, in spite of the curious math) and $42 and $32 for the spray.

According to Iranian Ambassador Ardeshir Zahedi, last year's Halston gala, complete with Halston fashion show from Garfinckel's, raised $40,000 for CARE.