But for stability, it would need to be high up the back - I can't physically hold the phone stable and have my index finger sufficiently on the back for sensoring unless it's at least halfway up and central.

Also, having just tested, this completely blocks thumb usage - there's just no way it can be done!

I don't get it. I can comfortably hold my phone pinched between my thumb and index fingers at just about any location. The finger on the bottom just needs to be a little closer to the center of gravity. A sensor where my 6+ says "iPhone" on the back (about 1.5" above the bottom) would work fine. But even if the sensor was up high, so what? It'll still work if your finger is between the phone and the reader.

As for the thumb, just flip your phone over. Although this would suck if you need any visual feedback, obviously.

> I can

Yeah but you != me. And I am by no means the least dextrous person I know.

I can't picture how a lack of dexterity would prevent this. But even if it did, we're back to my original point about range: it's OK to have your fingers in between, it'll still work.