I for one would welcome an export. I haven't played TBRB in a while (though I've meant to, been busy the past few weeks), but there are some things I just like about RB3 better. I don't like playing non-pro drums anymore, and I enjoy being able to bounce back and forth between multiple artists and having a huge set at my disposal under a single disc. Heck, my TBRB DLC purchases were tempered by the fact that it wasn't the main game (otherwise I'd have snagged the full Abby Road album).
Now, I do understand why it hasn't happened yet, but I do think that we'll eventually see an export. At this point, it's probably the only way we'll see more Beatles DLC. Even then, it would be problematic similar to the CTP2 issue, though in this case it may simply be the best option available. It would also probably require them to create RB animation tracks for all the songs, since I doubt the Beatles animation track is compatible.