After having a chat with some guys developing the open source radeon driver on freenode, and with the reverse engineering of GPU-Z I found where the ATI ATOMBIOS is located.
Pretty simple actually, you take the BAR2(Base Address Register #2) and XOR it with 0x80000 afterwards you AND the result with 0xFFFE0000.
In practical terms
atombiosaddress = (BAR2 ^ 0x80000) & 0xFFFE0000.
Also, the VBIOS of the primary card can also be located at physical address 0xc0000 like radentool has specified.
EDIT: I did a binary comparison of my dump of my card vs the one from GPU-Z and mine seems to contain different data at certain locations. So not quite there yet.
Something interesting I discovered is that the location of the ATOMBIOS on the primary card is not always available at the address using the formula above, the address is valid, however the GPU-Z driver does something to make the card map the BIOS to that location, after it's done the address is valid, just doesn't contain anything. It works on non-primary cards though.
