OK, update:
I've tried the following:
* InterAct card: AR program crashes when trying to write to it.
* AR card: AR program can write to it, but big files get corrupted (vmlinuz, rcinitd).
* MS card: AR program can write to it, the files do not get corrupted.
Then it's the matter of save game:
* Set of three save games (emergency, restart, install) from codejunkies.com: Hangs the Xbox.
* Save games from xbox-linux.sourceforge.net: Not recognized by AR program.
* Save games from xbox-linux.sourceforge.net, repacked correctly: Works, boots Linux on the Xbox.
"Repacked correctly" means to put the numbered directory in the "original" directory (since I have a non-updated MechAssault) in a 'UDATA' directory, making three copies of it, and then making zip files of it but only with one of the lowest-level numbered directory in each. That is, unpacking the three zips in the current directory should give you this structure:
MechAssaul_EmergencyL/
MechAssaul_EmergencyL/UDATA/
MechAssaul_EmergencyL/UDATA/4d530017/
MechAssaul_EmergencyL/UDATA/4d530017/48D918775FFB
MechAssaul_InstallLin/
MechAssaul_InstallLin/UDATA/
MechAssaul_InstallLin/UDATA/4d530017/
MechAssaul_InstallLin/UDATA/4d530017/E8FF68C9193B
MechAssaul_RestoreDas/
MechAssaul_RestoreDas/UDATA/
MechAssaul_RestoreDas/UDATA/4d530017/
MechAssaul_RestoreDas/UDATA/4d530017/3F0EFB83E4C