I suspect that the problem is not the P45 chipset, its the size ~120KB of the new O-ROMS (in uncompressed data) that no the fit the rezerved 90K ROM resulting in the board reading at the wrong address from the ROM module.
So the only chance to make it work for P5 motherboards is to somehow shrink the size of the o-rom (?! abbreviation the txt strings inside) to 90K.