I don't know about the other issues involved, but one way to make a rear-view mirror practical to use (bear in mind, this may not be practical to implement for programming reasons, for all I know) would be to model it as a convex (fish-eye) mirror. Maybe frame rates could be saved by simply faking the mirror view, or even making it a bit like a cross between the attitude indicator and a tail-warning radar.
What I have in mind there is that land and sky in the mirror would be simply depicted as green and blue respectively, and the mirror woulduse generalised shapes for single engined, twin engined and four-engined planes (3-engined go either as twins or fours, depending on size! :-} ). ALso have a limited number of aspects for each generalised type shown, one per 45 degrees, say. And yellow flashes if its shooting at you.
Perhaps with those limitations it'd be enough to give you some idea of whats happening behind you, without hitting fps too much, and also without using a mirror that's too oversized?
Esme