Detailed MechInstaller howto and easy-to-use MechInstaller FATX images available. Download the FATX images here and read the tutorial here. For Windows Users, there is a simpler addon tool too.

With this FATX image of the MechInstaller you can now easily transfer the hacked gamesave on a PC USB stick under Linux so your xbox can load it (no need to add fatx support to your OS).
All info on how to do this is in the tutorial (by Michael Steil)

update:
Xbox-Linux added the win32 'addon tool'. OzXMemory (by http://www.ozxchip.com) will allow to write to your PC USB Pen with FATX support. So you can just move the MechInstaller-FATX-version directly from your PC to your USB Pen. (must be FATX-formatted on your xbox first ofcourse)