Somehow, I've managed to forget where I downloaded Quake II for the Zero. Would someone be kind enough to link me? Thanks!
-EDIT Someone was nice enough to explain why I thought I needed another download in the first place. It's not working on the latest firmware version yet.
-EDIT Member Vato pointed out that with a little cajoling, Quake 2 WILL run on the 5-5 Firmware. I have pasted his instructions here so that this post will continue to be relevant to this section.
"- on your pc, use 7zip to extract the contents of quake2.opk to a folder (this is also a good spot to add the full games pak1.pak and pak2.pak to the baseq2 folder, if you happen to have those files available).
- drop the un-opk'd quake2 folder somewhere on your sd.
- fire up the GCW-0 and tap select to bring up the menu. tap A to 'Add link in...'
- navigate to the quake2 folder, highlight sdlquake2 and press start to confirm.
- that's basically it. find the sdlquake2 in gmenu, boot 'er up and start the slaughter fest. " -Credit to Vato
Also, as Vato pointed out, the Quake 2 opk may be downloaded at https://docs.google.com/folderview?id=0B9EgGumkJaabSE9TaEdFbVVBSlU#list
EDIT: For the USB Multiplayer information, see Page 3, Reply #36.