Street of Dreams
This 1988 documentary starring Tiny Tim pretty much speaks for itself and is definitely worthy of the Rifftrax treatment
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tsarstepan commented
If you can edit the title to this suggestion in order to keep from confusing voters as there's a film that came out that same year with an almost identical title.
Street of Dreams (1988) is an action thriller starring Morgan Fairchild.
https://www.imdb.com/title/tt0096182/referenceAs opposed to the documentary in question:
Tiny Tim's Street of Dreams (1988) or
Street of Dreams - Musical Mirror Maze (1988)And as far I know, there hasn't been a feature-length documentary, let alone a 5.5 hour one. This could be their most ambitious riff ever.