I followed the lovely guide to set up AFP/Netatalk/Avahi from the wiki, and was able to get one of my Macs to use the resulting share without issue.
However, when trying to configure this on a second Mac, it failed. When configuring the settings in System Preferences > Time Machine, I go to add the Time Capsule (OpenWRT) provide username & pw, and then the dialog dismisses without an error, but the backup location is not shown in System Preferences.
I looked in Console.app on the Mac, and found the old "Failed with Error 57" message. I found a similar topic on the forum here, but that seems unresolved. There is a post on the Apple Support Forum that seems similar, but it suggests changing the setuid bit of the permissions in the Backups directory, which is unset in my case (per the OpenWRT guide).
Anyway, I know it's a long shot, but I'm hoping someone will be able to chime in with some suggestions (@Mrengles perhaps?)
Thanks for your help.