The router was working fine. That is the part I want to emphasise, because everything that follows was entirely my own doing. It routed packets, it served DHCP, nobody in the house had complained in months. So naturally I decided it needed OpenWrt.
The flash went in cleanly. The reboot did not. What I got instead was a device that powered up, blinked its LEDs in a pattern that meant nothing reassuring, and then sat there refusing to acknowledge that a network had ever existed. No web interface, no ping, no SSH. A brick with a power light.
I had, of course, not checked whether this particular hardware revision was actually supported before flashing it. The wiki listed the model. It did not list the "v2" stamped on the underside in letters I noticed approximately one reboot too late.
What saved it was the serial header on the board, four pins I had nearly not bothered to identify. A USB-to-TTL adapter at 115200 baud, a U-Boot prompt that appeared when I interrupted the boot, and a TFTP recovery of the original firmware. Twenty minutes of holding my breath and the thing came back from the dead, restored to the working state it had been in before I got involved.
It is back to running the stock firmware. I have learned my lesson, which is to read the hardware revision before flashing, and to never trust a device that does not expose a serial console. The OpenWrt experiment can wait for a router I have not yet grown attached to.