The custom camera stuff is all there already, I think. So to make a rear view mirror, AA needs to do a couple of simple things.
Allow assignment of multiple viewports to the same player
Allow custom sizing of viewports
Allow the HUD to be toggled per viewport
Custom sizing can be something simple like proportional sizing, ie lower numbers mean smaller sizes. So if I set viewport one at size 2 and viewport two at size 1, viewport two would be half the size of viewport one.
Assignment of multiple viewports might be trickier, based on reading the config options. Config options have you assigning a viewport to a player rather than a player to a viewport. If you can assign a player to a viewport, then you'd just assign the same player to two viewports (or more, or whatever).
Then I could use camera options in the viewports to set one up that points backwards, and I'm done.
