A proposed postage stamp honoring Elvis Presley might well raise questions of taste, but not questions of constitutionality, or at least most postal patrons would assume.

So a press release from the American Civil Liberties Union with the headline ''A.C.L.U. Says Elvis Stamp Unconstitutional'' appeared a bit far-fetched. According to the press release, the A.C.L.U. was prepared to go to court to block the stamp on the ground that to bestow such an honor on ''The King'' of rock and roll would violate Article I, Section 9, clause 8 of the Constitution. This is the ''title of nobility'' clause, which provides: ''No Title of Nobility shall be granted by the United States.''

Although the release sounded like the real thing, it was a spoof, drafted by Arthur B. Spitzer, the legal director of the A.C.L.U.'s Washington office and distributed to some.

Yes, civil libertarians do have a sense of humor, Mr. Spitzer said, adding: ''I guess there might be people out there who don't know that.''