If XBC were to ftp into someones box to do a checksum on the cachefiles, it would first, have to have their xboxs ftp address (different for everyone where in halo its always the same) it would then need a username and password, then it would have to scan the cache drives and find the maps, verify that it's scanning the correct maps, check them, then report back. Furthermore, since you cannot ftp into someones box while running Halo, if they have a trainer on their xbe, or if they load HCE before halo, no one could know. Because Halo only checks for desyncing with the important things that effect both boxes, (I imagine its for bandwidth reason, as well as not wanting to code it all) it is impossible for one box while in Halo, to check another box for hacks (also in Halo.) UNLESS, someone were to rewrite the game to check for KNOWN hacks, such as glasswalls, zoom hacks, and glowing characters, or other general differences then cause a desync if this is found, AND BOTH PARTIES had this patch applied to their game. If one person didn't have it, it would either desync immediately, or do nothing. Therefore, this hack is unfeasable.
Of course... Just a month ago it was unfeasable to browse the web from your xbox.