The best way to accomplish this is to get Linux to boot on the Xbox 360 and then use Mac-on-Linux to run Mac on it.
This is what I do with my Ibook so that I don't have to dual-boot Linux and OS X. I just start up OS X in Linux any time I want to use it.
Then the chipset and everything is immaterial since all the hardware interfaces are provided thru Linux-based intermediate drivers.
That's a lot of work for not any benifit. You could boot up it theoreticly, but no 3d performance, probably no network, and you have to fight your own computer to use it.
Microsoft probably learned it's lesson from Linux on the first Xbox, they probably have significantly more barriers to FOSS software then before.