When the dress of the evening is black tie or burnoose, you know it's the Textile Museum's spring benefit. Last night at the museum, "A Caravan Though Arabia" was a walking showcase of, as the invitation read, "Exotic Middle Eastern Costume." The drink was arak, the food was stuffed mussels, lamb stew, hummus and other Mideast specialties.

The honorary chairman, Nouha Alhegelan, a lawyer and wife of the Saudi Arabian ambassador, Faisal Alhegelan, wore an exotic embroidered pink coat over full pants. "It is an antique Lebanese costume from a friend of mine," she said. "I have done many benefits this year for the Lebanese, but this is the only one for the Americans." The ambassador had the flu, so the Alhegelans' son, Hisham, escorted his mother.

Carolyn Deaver wore a handpainted chiffon dress: "This is as close as I could get to exotic." She said her husband Michael, Reagan's deputy chief of staff, had spent the day in Miami with the president, who spoke there. "And I suspect my husband is home now with his feet up," she said.

Deaver admitted she had a hard time keeping up "with just what town he's in. Like the day we woke up at Versailles, had an audience with the pope in Rome and dinner at Windsor Castle in England."

Deaver added that she was looking forward to the after-dinner Arabic coffee. "The last time I had my fortune read at the Turkish Embassy, the fortune teller predicted I would get to go on a large body of water--and I did, with Queen Elizabeth."

Jane Weinberger, wife of the secretary of defense, was another administration "single" last night. Her husband Caspar was at the Bob Hope USO benefit at the Kennedy Center. The previous night, he had gone to the Clare Boothe Luce exhibit at the National Portrait Gallery without her. "We just can't both go to everything," she said. "So we pick and choose. I was on this committee, so I wanted to come especially."

Harry Gray, chairman of United Technologies, was the most popular person there with the Textile Museum members. His company picked up the check--about $25,000--for the event, so the money raised by the sale of 280 tickets at $150 per person could go to the museum.

"We have some projects in Saudi Arabia," said Gray. "We'd like to have more."

Raymond D'Argenio, a United Technologies official, added, "We give $3 million a year to education, $3 million to health-welfare and $3 million to art. So this is not such a big contribution for us. Besides, the Textile Museum is a gem."

"We'll spend the profits for new storage facilities for our Islamic collection," said Patricia Fiske, director of the museum.

Sheryl Ameen, event chairman, apologized because the camel sculptures at the door, though handsome, had two humps. Saudi Arabian camels have one.