Guys, I just remembered something interesting: With the Live File System option for burning discs in Vista, you can burn a few times to a disc before it's full. Do you think the LFS is compatible with the Xbox 360? I'd love to try something like this for a disc of demos, adding demos and content as I find them...
Burn a disc in Nero and only finalize the track and not the disc, that way you can add more stuff later. That might work in the 360, try it and see.