I actually reconfigured my ZFS this week, scrap and build from scratch and ran into problems with Time Machine too. Not sure if I did this last time and forgot, it looks familiar. I'll document it this time!! haha
I simplified the instructions a little for newbies to understand easily. These worked perfectly for me.
1) Make sure you have Server App and liberate hack installed
2) Open Server App
3) Go to File Sharing
4) Add file system you would like to use for Time Machine
5) Open Terminal
6) Type "sudo serveradmin settings sharing:sharePointList:_array_id:/Volumes/My/Backup/Location:isTimeMachineBackup = yes"
7) Quit Server App if open
8) Open Server App and Turn on Time Machine
9) Should now be available as a Time Machine Destination