In the December 2002 edition of UK film magazine Empire, they have a feature called The 45 Greatest Rock 'N' Roll Moments in Movie History. At number 23:

23 Prince And The Revolution, Purple Rain
Purple Rain (1984)

SCENE BEGINS AT 87th MINUTE
After seemingly neglecting a demo tape of a song given to him by his backing singers, egocentric music genius The Kid (Prince) comes good at the end and unveils the song (which, of course, Prince wrote without help) in all its glory to a rapturous response. Remarkably, the audience seems to know the words.

DID YOU KNOW?
Jay & Silent Bob Strike Back ends with a nifty parody of Purple Rain, complete with cameo perfomance by Morris Day And The Time, Prince's rivals in the movie.

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So there you go. I'm not sure that the audience knew the words, apart from maybe the "Purple Rain, Purple Rain" refrain. Which can't have been all that hard to pick up at the performance! And was that really a parody of Purple Rain in Jay & Silent Bob Strike Back? I mean, I'm good at spotting the most minute Prince parodies, and the last scene in J&SBSB seems to me just a chance to get funky rather than parodying PR.