Success! I have the OpenWRT image installed and booting fine.
So here's a summary for anyone else (confirming what @pavelgl already said):
- Doing a firmware update from the TP-Link web interface doesn't work, even after adjusting the hardware identifier in the v5 image and updating the MD5 checksum
- Setting up a TFTP server to load an image during boot doesn't work either -- I couldn't see any relevant traffic on the ethernet port at the boot stage
- Serial works, but in my case I needed to configure PuTTY to 120,000 8N1 instead of 115,200 8N1 in order to be able to read the U-boot boot sequence and to enter "tpl" to get the prompt. (Then switch back again to 115,200 if I want to read the kernel boot messages after that.)
- There are no "loadb" or similar commands for loading the image over serial
- However there is "tftpboot" which can be used to load the image from a TFTP server
- Loading the Ashus 19.07.7 image for TL-WA901ND v5 worked fine, as indicated above
- The flash address is the same as on the TL-WA901ND page, i.e. you need to load to 0x9f020000
Thanks for the encouragement and hints.