Never mind - I took another look planning on measuring my available space. The rear shade is much closer to the top of the rear windshield than the bottom, to the point where I don't think anything will clear up there. The back 'deck' behind the seats is mostly that fuzzy Alcantra stuff (where it isn't giant speakers). But there are 3 "top tether" plastic covers right behind the rear set headrests, for car-seat attach. I think I might consider mounting a rear cam to the central one of those, if I can figure out the wire routing to get to the front cam. It's really going to be my best and likely only option, although placement there and looking somewhat 'level' will cost me some downward angle view - I wouldn't be able to even see license plates of cars behind me.
Maybe I need to consider an external cam if I really want rear, down by my own plate. I do have a nice rear cam of course - too bad car makers don't offer ways to just tap into their existing video lines!!