The boards finally arrived, and I got a chance to test the battery holder:

The tabs can just barely be lifted enough to insert the coin cell in place, but it's extremely easy to break them. On the other hand, they hold the battery in place quite well, so I think this will work, I just need to make the thin parts of the tabs much longer.

Having tested that part, I went ahead and assembled one board, with a regular battery holder this time:

I have the bootloader flashed, now I need to prepare the board definition for CircuitPython and get display to work on it.