Why not just use rsync? With my experience, I'd rather go with that than any solution based on time machine, especially over Wan, as it is quite unreliable if the connection isn't perfect.
Otherwise, as long as you can mount the share as smb, you should be able to use it for time machine, so anything like freenas, truenas, unraid, open media vault should do the trick.
Thanks for the fantastic options I never thought about. I just don't trust the USB external drive. I've had nothing but bad luck with it.
Thanks again!
Rsync is great, but unless you are using a system on top of rsync (eg. rsync-backup) it only gives you a single copy which isn't the best option for backups.
For example, you accidentally change a file but by the time you notice the version you want has been overwritten.
FreeNAS can do it (either via AFS or SMB). Or you can just set up a Samba server with the right magic incantations and use that network share to do Time Machine. I use FreeNAS since I already have it on my network. I gave each Mac its own ZFS volume so I can limit the size independently.