The boards arrived a couple of weeks ago from JLCPCB. I fired off an order to DigiKey for the dome buttons...had those a few days later. I stuck some buttons onto a board with some tape, and then tried dfu-programmer to see if I might get away with just plugging into a USB port to do the initial setup...no dice. Good thing I brought the ISP interface out to some pads on the bottom. The snag there was that just sticking some bits of header onto the business end of a USBasp was insufficient to make good contact. Looks like I'll need some sort of pogo-pin adapter to program the boards.
I found this 2x3 pogo-pin adapter that turns out to be perfect for the job. Orient it properly, push down for good contact, and fire up avrdude. With the software image, at least a minimal EEPROM image (twenty nulls), and the fuse bits set right, everything fired up. The serial configuration interface allowed me to paste in some passwords, which would then get sent on pressing the appropriate button.
I knocked together a case design in OpenSCAD and printed it out...good fit, but 1 mm thickness is probably not quite enough for durability, and I forgot to include a keychain loop. I've parameterized the case thickness so I can dial it in wherever I need it...will try 2 mm next, but first I need to replace the part-cooler blower on the AM8 in my office, as the one in there now has crapped out.