Oh, maybe I should've added that since the bios did boot we know (although I was quite sure of that already due to the MCPX emulator tool) that there has been no MCPX-changes, which also means we _can_ create a modified bios.
It would still be good if someone with a v1.6 could, as heinrich suggested, flash an LPC modchip with the original 5713 bios, ground D0 and try to boot. If that works, we know that there has been no additional LPC-changes. Watch out for a possible loss of power and ground as in v1.5 though.