There's certainly precedent for it. When I used to live in the Philadelphia area, the argument over whether the black nationalist Mumia Abu-Jamal shot the Caucasian police officer Daniel Faulkner was an extremely hot topic. Proponents of Abu-Jamal's guilt and execution often cited Officer Gary Wakshul's testimony that Abu-Jamal said at the hospital, "I shot the m*therf*cker, and I hope he dies." On the other hand, Wakshul's testimony contradicted an earlier report filed by Wakshul himself, which stated "the Negro male made no statement," mostly like because Abu-Jamal was comatose from a bullet wound at the time. In this light, Waskshul's testimony about Abu-Jamal calling a white cop a "m*therf*cker" may have been Wakshul's attempt to put "black street slang" falsely into Mumia Abu-Jamal's mouth in order to sublimininally portray Abu-Jamal as a scary black nationalist cop-killer.
A similar force may have been at work when Sgt. Crowley attributed "I'll speak with your mama outside." to Henry Louis Gates. But then again, is it possible that Henry Louis Gates actually said that? Part of me wants to believe he said that anyway, because I don't necessarily think it reflects badly on him, and I think it would be such a hilarious crotchety and curmudgeonly thing for a professor in his sixties to say to a cop. I mean to say, if you were harassed on your property by a cop, don't you think the cop might deserve a little sassmouth?
Then again, did the reference to "your mama" have anything to do with Henry Louis Gates and his career as a theorist of African-American literary criticism? Gates may be most well-known now for doing Oprah's genealogy, but the book that made his academic reputation was The Signifying Monkey: A Theory of African-American Literary Criticism, which stressed the importance of African-American vernacular traditions like playing the dozens and insult games involving "yo mama" jokes. As the commenter "mistersquid" noted at Metafilter,
Henry Louis Gates references Clarence Major’s Dictionary of Afro-American Slang, which compares signifyin(g) to the “Dirty Dozens,” “an elaborate game traditionally played by black boys, in which the participants insult each other’s relatives, especially their mothers. The object of the game is to test emotional strength. The first person to give in is the loser” (qtd. in Gates 68).
So, there you have it. Either the cop was "blacking up" his report in order to make a African-American Harvard professor look bad, or Henry Louis Gates was engaging in literary criticism mind games with the cop who tried to arrest him. Either way, it's indicative of the fascination that African-American vernacular English holds for both blacks and whites in this country.