Ok, I lied to you, I actually did try Back-Powering an A+ in 2015.
I just tried it again, yep it works no problem at all! This is the older 256MB model using Stretch. A little slow on a 256MB! I do not know if there were any changes to the newer 512MB models. If not they should work too.
I guess I now have to try a Zero. I need to find a short A to MicroB cable. Cable length, as in short, is important if you are going to successfully try back-powering.
Ok the USB MicroB to A adapter into my A to Mini and the Zero W booted right up with back-powering. A single cable solution would be much better for this! So I would need a MiniB male to MicroB male cable.
I found Back-Powering to be safe. You back-power at your own risk!
You CANNOT SAFELY power any Raspberry Pi by back-power and it's own power connector simultaneously. Use ONLY one OR the other, NOT BOTH!
Some Raspberry Pi's are protected from back-powering by a diode.
Might be time for a new back-powering discussion with the proper warnings about how it is done safely!