I was trying the same thing with MW2 map packs, seemed like it would be easier to change the region of the game than the region of DLC (since we have keys for Games, not for any PSN stuff). When DLC is installed, it goes into a BL*S00000 folder inside hdd0/game/ for whatever game it's intended for. Trying to change that is a lot harder, I've tried renaming the folder or moving them so it's the correct BL*S code, but they weren't detected because there is so much verification (making sure you paid for the content and that it's activated). Then there's the act.dat and .rif files which are linked to your user profile (located in hdd0/home/000001/exdata). I just tried them in a hex editor (Rockband DLC) and it does have a region code in there so that'll need editing, but I'm not sure if it'll work after modding, but I'll try later and report back.
You'd need CFW on your PS3 to do it, but I think changing the PARAM.SFO and the EBOOT.BIN (and anywhere else it says the BL*S code) of the EU copy of RB3 to the correct region could work and it's fairly straight forward to do, then you don't need to change all your 900 DLC files. TBH, I think either importing the US version or going the CFW route are the options you have.
If you're not on 3.56 you could maybe try installing CFW, mod RB3 to try make it work and then update if you want, once it's working
Just my take on it.. I might be wrong!
oralphie: Will this allow chuck norris to play backups??? PLZ HELP!!!