I admit it, I've been slacking on this. Of course they don't make it easy on you, what with DLC coming out every week and all. Now that there are more than 1000 songs available, well, it's a bit of a challenge.
Anyway, I've created a new Google Doc with most of the songs and Rock Band 2 difficulty levels. (Remember, RB and RB 2 have different systems for difficulty. Also, IMHO the RB 2 difficulties are less accurate. Take that for what it's worth.)
There are a number of songs with no difficulties yet because I entered only the song info, knowing I'd have to come back to them. This is still a work in progress. I'm down to the Rs with just the new Nirvana and Night Ranger songs to go back and fill in.
Finally, there are some songs in the game itself that I haven't marked, and then there are the songs from the first game that you can import. (I deleted some of those, so I may not have ratings for everything.)
Oh, one more thing: a few songs only use three instruments (one song with no drums, three songs with no vocals). For those songs, I removed the averages from any column with the part that is missing, so they'll be at the bottom of the list (because the full band can't play them).