Thanks xiayang for the pointers. I had to hunt around a bit to actually get all the pieces because git.ee and Google Translate and the fact that some of the branch links weren't working (even though they were there when cloned), lead to some fun detours.
I ended up having to boot in service mode from a TF (microSD) card that had Armbian flashed on it and then nuking the internal mmcblk0 to force MaskROM mode, but once I did that I had pretty good luck with both the Windows and Linux versions of the tools to flash the rk3399 loader and the latest Armbian images. I had one device that was acting a little funky but I finally realized that it was a powered USB hub causing backpower into the SBC that kept it "up" when trying to reboot but the trickle of power wasn't enough to let it really boot, so it got into a pretty bad state. Switching to a smarter programmable hub that doesn't backpower made the next couple devices I converted behave much better, though the first one I did still acts dumb at times.
What finally helped was finding these two links (still require Google Translate), but the first references a link that points to a cached version of the README that actually has all the images that had trouble loading on git.ee, and it also has some handy shortcuts depending on what you want to do with the devices.
https://mozi1924.com/2024/06/08/am40_armbian_hassio/
https://am40.cache.cloudns.org/#smart-technologies-am40
