Reminiscent of this scene from the 1981 teen-slasher parody, Student Bodies:
> Announcer: Ladies and gentlemen, in order to achieve an "R" rating today, a motion picture must contain full frontal nudity, graphic violence, or an explicit reference to the sex act. Since this film has none of those, and since research has proven that R-rated films are by far the most popular with the moviegoing public, the producers of this motion picture have asked me to take this opportunity to say "Fuck you."
> [the MPAA R-rating logo appears on the screen]
Fear of a Black Hat, a rap mockumentary in the same vein as This is Spinal Tap, begins with a warning about the language in the movie that's essentially them repeating the offending words three or four times.
Took a bit of poking, but: https://www.youtube.com/watch?v=KgDQ5z7bNPs