It appears there's no way to edit the Panelizer content/layout/settings for non-current node revisions. I've done a little digging and added some menu callbacks, etc. It seems the key limitation here is that panelizer_panels_cache_get() doesn't differentiate based on revision. Am I barking up the wrong tree?
I'm continuing to look into this in hope of rolling a patch but perhaps someone with better architectural knowledge of panels/panelizer can weigh in on the best approach.