You're right.. The bios is implemented thanks to an x86 assembly language and C.
There's no reason to look into the MS code because all the access to the pci ports ide, and memory are standard with all the x86 plateform.
However there is an issue with the nvidia card which contains also the network chipset.
For that, some developers have reversed enginneered the nvidia driver and create a new one under GNU licence : forcedeth.
If you want more information, have a look at the cromwell distribution of Linux for xbox.
Hope this helps