There are several hints here on how to get Time Machine to backup to an unsupported AFP share, but I found no mention of how to restore from such an unsupported share. After spending some time on Gooogle, I found this page. The essence of the solution is that once the installer has booted from the DVD, one has to go to the Terminal and mount the AFP share manually:
$ mkdir /Volumes/TimeMachine
$ mount -t afp afp://user:password@afpserver.local/ShareName /Volumes/TimeMachine
Quit Terminal, then enter the Time Machine Restore utility. The unsupported backup volume should then show up.

