I would hope MS started thinking about this early on in the DLC process - not just Rock Band stuff - and has something in place that will allow both full backwards compatibility and DLC transfer. Actually, if anything Rock Band is one of the games I would be LEAST worried about here. Provided the franchise still exists then, I'd assume Rock Band 5 (or whatever) would be compatible with all the old content - if nothing else we could redownload it on the new gen console, at least.
I'd be more worried about content for older games that no one pays any attention to. If any "work" is required, I can't see them making the free bonus cars for Burnout Revenge available, for example.
The backwards compatibility issue shouldn't be a problem this time around (hopefully)...it has been a few years since I read about it, but going from fuzzy memory, they made some mistakes with the rights over the hardware of the original XBox, which made it cost-prohibitive to ensure full backwards compatibility between the XBox and 360. I think they have more control with the 360's hardware rights so it will not require software emulation like we have to have currently. Someone correct me if I'm wrong on this, it's been years since I read those two books about the creation of the XBoxen.