Can Time Machine on a server use quota?
Solution 1:
Looks like you're trying all the right things. I do this with a Drobo for a small number of Macs now. I have a 2TB volume, but I don't want that to fill up with old TM backups so I just use the sparseimage trick and it works great.
It is unclear why this is getting reset for you. Perhaps you should try this again.
I found these instructions useful: http://www.somelifeblog.com/2009/02/fixed-time-machine-backup-to-network.html