Rather than reproduce everything here, may I direct you to the Apple Support Communities thread at https://discussions.apple.com/thread/7509348?
I wasn't sure it was a ZFS issue when I first posted the query, and I'm still not sure it is. However... If anyone here can offer guidance I'd appreciate it.
It used to work fine. It still works fine, except that since Server 5.1, servermgrd goes into a tizzy when it looks at the TM destinations and sits at 100% CPU forever (well, days).
Basically, I backup the family machines using Time Machine to a dedicated server. Each machine has its own TM Destination, which is a ZFS filesystem, which I snapshot daily.
The ZFS file systems are mounted under their own directory (/Wells), rather than than under /Volumes.
The whole server machine is then backed up offsite using Backblaze.
TM is a wonderfully convenient and painless way to back up the family machines. ZFS then keeps them safe against hardware failure (OWN Thunderbay 4, disks in ZFS mirror) and Backblaze maintains an offsite copy.
OS X version: Mac OS X 10.11.4 [15E65]
OS X Server version: Server 5.1
ZFS version: SPL: 1.4.5-1, ZFS:1.4.5-1