Ok well, I've tried a few things and it always leads to the same thing onscreen: input signal out of range. I have to plugpull and reboot it every time.
Trying to change HDMI modes with a10disp leads to a message saying pretty much every resolution is reported by the driver as incompatible.
Oddly enough, the very first screen that appears on power-up correctly auto-sets to 1024*768. After it goes into Armbian though, it changes to 1920*1080. So obviously Armbian isn't "reading" the screen right.
How would I use fbset to add the unsupported resolution in the way you suggested?