Well, ok, after some attempts and NO help i managed to obtain the elf from the rpx and I replaced the rom (the header is now clearly visible) but now the makerpl gives me "Could not map in file FAKE.elf" Error 0x80070002.... any hint ?
EDIT:
Going further: now i get CAFE_ROOT is not defined.... maybe a "space in folder names" probelm... will dig into it...
EDIT2:
Finally got it to work (it was a "space in folder names" problem).
Dunno why nobody would like to explain me how to do that anyway I got it; will try to test a NES title soon.