If I were a modder, I'd try a MITM method. Attach the HDD to "some device" (Arduino?) and try to interrupt the boot process at various points until I'm confident I can start the console and switch to another HDD/USB key to keep booting but with my OS instead. And I could put a switch to revert to normal boot
Of course this would totally fail if the BIOS does more than a boot check. I'd have to adapt (hence the Arduino - or other, in the middle).
Even if the BIOS did do a boot check, your MITM method could work. If anything, that may be one method that modchips use! Say you used the Arduino (very useful device btw), and you figured out the entire boot process. You could have the BIOS verify the stock hard drive on an unmodified OS, and once the boot check passes, you could flip hard drives using an exploit of some sort, booting whatever you want. However, an easy fix to this would to do a boot verification, verifying the OS checked was the OS booted. That also can be bypassed using some complicated stuff though too. Read on.
You could have the arduino intercept whatever code or hash the BIOS will check during the Boot Check and have it stored. Then switch hard drives and boot from the custom OS, but emulate the other OS on top of it (or just the hash checked) and have it report that when the Boot Verification process would be called. After that check passes, you could then reinstate the custom OS and run it like normal. However to fix that too, the BIOS could run a Persistant OS Check, checking the OS signature every X amount of time. That can also be bypassed by masking the OS somehow using some exploit, and just freezing the original OS "check hash" in the memory, so that key keeps getting pulled while the custom OS runs.
There are a lot of ways you can go about bypassing BIOS checks. But there are a lot of ways to counter them as well. I wan't to learn how to program code so bad so I can hack systems myself, I just haven't taken time to figure out, 1. what language(s) would be necessary for exploit research and hack programming, and 2. actually learning such languages. It's something I'll eventually get around to, I just need some advice.
Or we're going on about it the wrong way. What about an XBLA program for mining?