These guys are probably a safe bet for older stuff: http://customsforge.com/topic/15575-classic-rock-coalition-home-page/ I've done sone 50's-70's stuff too (link below)
Bob Mould (Husker Du, Sugar) answered this for me by covering a song solo that he wrote and recorded with his band. http://www.avclub.com/video/bob-mould-covers-sugar-53053
I think it would probably be easier to have an "out of sync" flag. Otherwise, it would take forever to go through all the customs posted and then you would have people avoiding the ones that haven't been gotten to because they haven't been flagged as "well synced" yet.
I like DD and have included it in the customs I've done. If I download someone else's that doesn't have DD, I add it. If that doesn't work I throw it in the trash. I use mostly non-stop play and I'm focusing more on the mechanics of playing (switching chords cleanly, picking cleanly, coordination) than learning particular songs and I find DD helpful. I don't care if experts consider it an inefficient way to learn.
That's a good question...where ARE the major bands? There's no DLC yet for the Raspberries, The Lemonheads, Matthew Sweet... I need my power pop dammit.
As far as 90's punk goes, Offspring and Green Day got all the attention but in my opinion the Swingin Utters and Rancid should have. Also, Social Distortion.