I haven't seen anything that would provide this functionality aside from using an external network storage system that you mount under OS X, such as a Snap Server (that is in fact the set-up I use).
However, I do know that it is possible to Script this functionality into a task that can be run from Cron on a regular basis. I'm currently trying to put something together in Perl that would accomplish this task, but I will be the first to admit that I have only recently begun to learn the language, so my attempt will not be coming quickly. I'm sure there's someone out there that could probably write something to this end, at least to give a notification if someone is over the quota, in a very short time. Bribe them, you never know what'll work.
Ciao!