Well, after looking a little bit more into the firmware of the drive I used, it's clear that my understanding of RPC-1 firmware was a little bit crooked.
RPC-1 only tells the region-setting of the DVD-part of the drive to stuff it; the BR-part is still "locked" to one particular region through the firmware of the C-200, which can be switched 5 times. After that, the region set last is the region you'll be stuck with.
This leaves me to try and find some firmware which does what I want.
As I also have a PS3 which has the A-region, I'm not in a hurry to do that, but I'll keep my eyes open.
Cheers, K.