I would love a Genius.com / annotation layer on top of these bills too. Just a dream I'm sharing out loud for no particular reason :) love govtrack in general otherwise!
Without commenting on the merits of that idea, I'll just say that I do not want to be the one who has to moderate user generated content.
Only if it is shared annotations is it a problem.
It need not be shared , think more like a public notion/ share point document with comments visible . I.e experts(users) can create their own individual annotated versions and share with others .
As long as there is no single version of the annotations , moderation is not needed