After shooting myself in the foot a few times (mostly running out of disk space to create the ext4 image) I’ve got a better build from the SeeedStudio release which has Docker built in along with a bunch of other things. So many things now, in fact, that I need to copy new images onto the H68K and run sysupgrade interactively on there rather than upload through the Luci interface which times out.
This is working really nicely for me now as my HomeAssistant server - all running in Docker, using an NVMe SSD in a USB-C enclosure for storage.
I did have a brief attempt to apply the H68K patches against the HEAD of the OpenWRT tree but it was non-trivial and I didn’t have the time to dig into the details of what was going wrong.