Rosalynn Carter, who took her sewing machine with her to the White House when she moved in, will share the cover of the October issue of McCall's pattern catalogue with Marlo Thomas. Both Mrs. Carter and Thomas, who were photographed recently at the White House for the cover, are wearing items made from McCalls patterns. Mrs. Carter didn't make the cropped blue wool jacket, beige silk shirt and soft plaid skirt that she wears in the picture. However, she still sews a little when she has time, explained a White House spokesman, but it is mostly "hemming skirts and stitching blue jeans and fixing Amy's clothes," said the spokesman.

Mrs. Carter, like her predecessor Betty Ford, who also appeared on a similar cover, won't be paid, but a percentage from the sales of the patterns for the items that Mrs. Carter is modeling will be given to the McCall's Life Pattern Scholarship Fund, which provides scholarships for women who want to get back into the job market. This catalogue will be out on August 1.

Will Dresden lead us the way of Tut? Which is to say, will we be running around in look-alikes of the Dresden treasures, now on view at the National Gallery's new East Wing? Certainly there is no denying the lavishness of the collection, particularly such exquisite baroque baubles as the gold and lapis lazuli girdle chain that belonged to Anna, wife of the Elector Augustus; the ruby, diamond and pearl pendant of a woman playing a lute mounted on a stag, or the rose-diamond garniture of the Saxon crown treasure.

A rash of Dresden-spawned jewelry is unlikely, according to John Bellivier one of the designers of David Webb jewels in New York. Bellivier, who knows the collection, says his customers for precious jewelry want more tailored items. "These (Dresden) jewels are stricly for certain types of dress," he says, "and our jewelry has to adapt to lots of different circumstances." He add, "There was some interest in hammered gold after the Scythian Gold exhibition at the Metropolitan, but that is over now. Maybe the Dresden jewels will influence the costume designers."

Costume jewelry designer Kenneth Jay Lane says the enormous Maltese crosses he did years ago were indeed inspired by the Dresden jewelry, which he has long admired. But, he says, the current jewelry trend is not for copies of Van Cleef or David Webb or other real jewelry, but enormous funny things like Kari Lagerfeld's supersized, obviously fake, junk jewelry shown in Paris in April. "If someone asks, 'Is that real?', then you are off the beam," says Lane.

Lane's current jewelry collection uses slightly oversized stones set in subtle, antique silver settings. "It's very wry humor, very subtle, a wink, you might say," the designer says. He likes these pieces worn on tweed jackets, cowlnecks, turtlenecks or even on hats. "It has more to do with Chanel or Schiaparelli in the 1930s than anything else," says Lane.

The fun of wearing costume jewelry has been ignored these last few years in favor of minimal gold chains. "Chains are a status habit that never made anyone look prettier," says Lane.He also likes the look of swinging drop earrings for disco dancing and big fake diamond or colored stone earrings to wear with tweeds and suedes during the day. "It's going back to chic dressing," says Lane, stressing that chic (being current) is quite different from elegant, which he sees as more classic.

Lane sold his Murray Hill townhouse to Liza Minelli, who moves in next month. He auctioned off most of his art objects and his skull collection, but he is leaving her the tent room and silver sink and chandeliers.

Steve Brody of Cadoro, who borrowed elements of Eqyptain jewelry for his designs in the swell of the King Tut show here, is high on junk jewelry, too. "We've been so big on necklaces for a while," says Brody, "it's just good business to move on to something else."

So what do you do for an encore after six influential and popular fashion exhibitions at New York's Metropolitan Museum of Art? That's the problem facing Diana Vreeland, special consultant to the museum's Costume Insitute. One possibility is a show focusing on dance pioneer Serge Diaghilev. Diaghilev brought the Ballet Russe to Paris in 1909 and revoluntionized the music, color and costume of ballet, as well as the dance itself. He used Stravinsky music, sets of Bakst and Benois; Picasso, Derain and Dufy did sets and costumes, and Poiret and even Coco Chanel did costumes for him. Now Vreeland is scouting Europe to see if there are enough surviving Diaghilev costume to make up a show.

"The influence of Diaghilev was complete. Paris changed," Vreeland said in a recent Rolling Stone inteview. The Met's current fashion exhibit, Vanity Fair, continues through Sept. 3. The next show, which will open in November, will be at the Met at the same time as the King Tut, the Temple of Dendur plus Splendor of Dresden exhibitions.

Jeffrey Bruce hasn't seen anything like it since the last time he was in Seattle - the way Washington women don't put on their makeup (or put it on badly). Bruce, a $100-an-hour makeup specialist touring to promote the new skin-care regimen, Catalyste, blames husbands who inhibit their wives from looking, "made-up," but whose own heads are turned by a well madeup woman. "Maybe they don't want other men to look at their wives," Bruce concludes.

Bruce has done the makeup for Jacqueline Onassis, Lauren Bacall, Eartha Kitt and Mariane Mobley. He says his least favorite client was actress Tovah Feldshuh, whom he found ill-mannered. "Only a very foolish lady would get on the wrong side of the person who does her hair and makeup," teased Bruce. "He can get even." (Bruce's warnings are not so idle. He once got so mad at a client that he put her false eyelashes on inside out.)

He has some advice for several prominent Washingtons: For second lady Joan Mondale, he'd recommend a longer hairstyle, trimmer eyebrows. "Playing up the eyes with color and using mascara would hep to soften the occasionally harsh look," says Bruce. For First Lady Rosalynn Carter he suggests "brushing through the hairspray" for a more natural look. (Even Farah Fawcett doesn't escape Bruce's critical eye. For her, he advises less white around the eyes. "Talent won't always win out - makeup will," he says.)

(The Catalyste line from Princess Marcella Borghese is at Garfinckel's.)

The latest word in T-shirts is also the longest: FLOCCINAUCINIHILIPILIFICATION. Oxford University Press is marking its 500th anniversary with a crisp white T-shirt that breaks the polysyllabic mind-twister into three hyphenated lines of navy type. It's the longest word in the famed Oxford English Dictionary, the reference manual that comes with its very own magnifying glass. Lest you forget, the word means "the action or habit of estimating as worthless." (By mail from Oxford T-shirt offer, box 10084, Stamford, Conn. 06904 for $3.49. Obviously, the company doesn't think it floccinaucinihilipolicatable.