Apparently, there is a new 'game' if you search Rock Band called "Rock Band Music Store." No, it's not an actual downloadable client to my knowledge (you know, like Halo Waypoint).
If you check it out, you'll notice that the DLC from both the Lego: Rock Band platform AND The Beatles: Rock Band are available in it. This should hopefully make some of your DLC purchases via Xbox.com a tad more convenient because it's all in one spot!
Rock Band Music Store