I'd suggest that $u/contentmeta.xbx is a manifest/catalog for any downloaded content, XBEs, et al. The filename also suggests as much
Content Meta-information - information about the downloaded content.
I'd speculate that the boot code in the DVD based XBL game's XBE consults this file very early in the start sequence - in case a replacement, updated, default.xbe is, in fact, included in the manifest. Given the number of HDD flagged XBL XBE's, this would appear to be a standard XBL library routine built into all XBL games.
Angerwould,
I've not yet look into the contentmeta.xbx, but at it's simplest it could simply be a list of filenames, along with their checksums, you could try replacing the $u/default.xbe with one from another game, and HEX pasting it's checksum into the manifest. This wouldn't get anyone very far, but it'd confirm the theory.