Spawn,
I had the EXACT same problem you're having, and I fixed it tonight. The absolute key difference for me? The TU files. And I don't mean download the newest one - I went to
http://xbuc.clanvids...tails=Rock Band and downloaded ALL of the RB1 and RB2 TU files, I think there were 7. Then, I put them all in the Cache folder on the root of HDD1 (really just HDD partition 3 root). Suddenly, all of it worked! I don't know why I didn't notice before, but originally, only my Weezer pack would run on my Jtag Xbox, and the pack came with its own TU file. This update must have specifically been the most current one when the Weezer pack was released.
The DLC packs come out periodically, so I figured 1 TU file wouldn't do it. Maybe you'd only need the newest one (which is now 6), but I went with all of them and am having great success.
PS, make sure that XM360 didn't make a "Cache2" folder, or a back up of your cache - that will confuse the game (or at least, it did for me). Also, as far as I can tell, the filename fix function on XM360 is only relevant if you FTP'd your files to the Xbox, as most FTP programs will shorten the name to send the files over. That function will fill out whatever part of the name got cut off. But, if you use a USB drive to transfer everything, you should never need to use it.
All of my DLC was placed in the 45410829 folder, and each within the 00000002 inside that. Lurkin was right about the sub folders - avoid those. Also, it should work being placed in the RB2 DLC folder (forgot the number, it ends in 69 I think), but I would just stick with the one listed above.
I'm no expert, but I've been screwing around with this damn RB DLC for days now. Hope this helps someone, and thank you, Spawn, for sparking the thread - my questions may not have been answered otherwise! And of course, thank you Lurkin and Node21 for all of your sage advice for us new guys.
PPS, all of my RB DLC is Yaris Swapped, all ID's are 0's. Good luck to anyone having the same problem!