The little bit I have read about these is that the more complex ones have two parts. The first part allows the user to interrupt the normal booting process and choose a boot device may it be a another boot PROM, flash memory card, or hard drive. The second part is alternative boot PROM. They probably hacked the one on the mother and using a BIOS chip from a motherboard with a similar chip set coded their own. Since the XBox is based on a PC platform, compared to Game Cube and Play Station
2 it probably would be easier to create a mod for the XBox but there is always all that nasty encryption surrounding the DVD-ROM drive.Unless you trying to start a business I would imagine it would be cheaper to buy one and possibly modify the mod kit.
Derek