So, I think I can do it simply with this jack:
Switchcraft : 1/4" 3 Conductor Open Frame Jacks
...that I found from this diagram:
Wiring a 2 X 12 Speaker cabinet
Anyone else have/used a jack like this? I emailed Avatar, and they will sell me a pre-wired plate for stereo/mono for $17.95 +5.95 SH. Fuck that
Don't be a pussy, just buy another cab.