There's more to the xDVD protection scheme than just a signed .xbe. If you look at the layout tools in the dev kit, you see all the reserved blocks for security and whatnot. It won't just boot a signed xbe off of a CD-R, AFAIK.
It reminds me of the PSX scheme of using some 'bad blocks' that cant be replicated on consumer equipment.
You could maybe temporarily mod with an xecuter, copy linux to the HDD, then remove the mod - but you can't replace/change the xbox BIOS in RAM so you'd never be able to boot backups or homebrewed stuff off of disc. So if you have to mod to copy an xbe to the HDD, why not just flash the TSOP anyways?
Now, what's my point? Ask yourself who would benefit from the key being released? Big-time commercial piracy. You'd need a real press to replicate xDVDs - the kind the big Hong Kong pirates have. So they can flood the streets with silvers of Halo that run on unmodified consoles. Believe me, this would be BAD BAD BAD BAD for all of us. Not only would it likely shorten xbox' life (like it did dreamcast's), but it makes every modder/hacker look like they're in cahoots with organized criminal elements.
A small hacker/modding community can be tolerated. Large-scale piracy wont be.
Anything the average guy like us could do with the key would mean modding, copying something to HDD, then removing the mod - kinda pointless.
Anyhow, that's just my .02