It's a little bit late, but I've updated the Mega folder linked in an earlier post with an image/imagebuilder for these APs that's compatible with the 21.02.1 release repos 
USB on the AP55/100 (non-C) doesn't work yet - I have a couple AP55 on-hand, but I've not worked out which GPIO actually turns power on/off. When I plug a USB device in, it gets powered on and off over and over again - i've not had time to go digging through GPIOs to work out what's going on there.
On the plus side, once OpenWRT is installed the RJ45 serial console on the AP55 works, so that's nice.