To clarify, the thing preventing DLC from working across different versions of the game is not region locking, but rather a consequence of how patches/dlc are distributed.
Every game has an identifying code, like BLUS-3434545 or whatever. The different versions of the game have different codes (BLEU for eu iirc? cant be bothered looking it up exactly.)
Anyways, the game is stored on the PS3 via this code, and the DLC/Patches are identified by the game code as well. So buying DLC for the US version won't work for the EU version because their codes are different and your PS3 has no way of knowing that they are the same game. (and besides, they might not be the same if they have localisation differences.)
Of course, there's nothing *technically* stopping them from coding into the DLC, "This DLC works for game codes BLUS-xxx BLEU-xxx BLJ-xxx" even if only people who own BLUS-xxxx are supposed to be buying that version of the DLC.
But that's not how the console DLC/Patching system works, and I don't know if MS/Sony even allows developers to break those rules, or how much effort and pain it would be to do so.