I have the same problem. We have running severall server (10.6-10.8). The Disk Full Notification ist a good Tool but we really want to disable it for the Time Machine Volumes. We have on every Server 2 Volumes for TimeMachine. 1) for the BackUp off the Server itself (external HD) an 2) for Backup the Clients.
I think it can be disable with an CLI Command but I have no Idea which command this could be