First, mod your Xbox to run software not approved of by the High Corporate Office in Redmond. Then, install Linux. Then, compile and install the Linux PowerPC emulator. Then, config it. Then, create a disc image of the MacOS Installer CD. Then, stick the disc image on the Xbox hard drive. Then, run the emulator. Then, install MacOS X.
Oh, but don't try any heavy lifting...the PowerPC emulation environment is so slow, running the installer takes around ten hours(!). On the good side, though, a display of geekery this excessive is bound to get you laid... :)