For lawyers who watch with excitement as the august rules of jurisprudence catch up with 20th-century morals, as the best law reviews begin to read somewhat like the Playboy Forum, the case of the late Alfred Bloomingdale and his mistress Vicki Morgan lives on.

Even if Bloomingdale had not been the heir to the department-store fortune and the founder of the Diner's Club, even if his wife Betsy had not been a close friend of First Lady Nancy Reagan, the future of the law suit that bears his name would still have much to hold their interest and affect their fortunes.

For the story of Alfred and Vicki may help take the rapidly expanding legal field of extramarital contracts -- called "palimony" in some quarters -- one immensely significant step further. No longer will attorneys for scorned women hold their fire if their clients have not actually lived with the source of their support and affection. Women dating married men, if they can prove they did anything for their man besides go to bed with him, now have a chance for large amounts of money, and even Vicki Morgan may get hers on appeal.

For that she can thank Janet Maynard, a former employe of the Santa Anita race track, whose own $2 million suit against Arcadia poultry rancher Harry Preister has already been up to the California Supreme Court and pronounced fit for trial. Like Los Angeles Superior Court Judge Christian E. Markey in the Bloomingdale case, attorney Marvin M. Mitchelson said, the judge in Maynard's case tried to throw out her claim for compensation after her affair with the rancher ended.

He ruled it was merely sex-for-hire, which Mitchelson and most other attorneys agree is prostitution and automatically illegal in California and many other places. But Maynard had done some work for Preister around the poultry ranch, and in the eyes of the higher court that made all the difference.

Mitchelson, who until recently represented Morgan and says he has about 25 cases of the same general sort under his wing, pioneered the field of palimony by winning a moral if not financial victory in the famous Marvin case. That gave contract rights to unmarried couples living together after his client, Michelle Triola Marvin, sued her former lover, actor Lee Marvin, and the principle, he said, is now recognized in 35 states. The Morgan case seeks to extend similar rights to mistresses who do not live with their patrons.

"It's a fundamental constitutional issue," said Mitchelson in an interview. "Do two people, unmarried people, have the right to contract for services that are not sexual?"

Judge Markey did not see it that way at all. The relationship between Morgan and Bloomingdale, "contractual and otherwise, was no more than that of a wealthy, older, married paramour and young, well-paid mistress," he found. The businessman, 66 when he died Aug. 20, paid $10,000 a month to the one-time model, now 30, over a period of 12 years.

Mitchelson said he does not know why Morgan switched from him to another attorney, Michael G. Dave. Dave said he cannot discuss in detail his plans for the case, but an appeal "is certainly contemplated." Markey let stand the part of the suit seeking compensation for Bloomingdale's written contract giving Morgan half of his estimated $1 million interest in the Showbiz Pizza chain and $10,000 a month for two years beginning last March as profits from a real estate development.

"This is an evolving area of the law," Dave said. "You are coming up against some established legal doctrines that have been in the law books for several years, regarding the sanctity of marriage . . . But they must be made appropriate to the 1980s."