Nothing except return warning messages via the quota daemon or something.
The quota daemon, if by that you mean "rquotad", reports things only when asked. It does not spontaneously send messages to NFS clients informing them that user XXX has gone over quota.
Have you seen traffic on the wire indicating that it's asked any time other than when the "quota" command is run? (It gets run, on many systems, at login time.)
It's straightforward for us to have our quota daemon report soft quota information. Modulo changing the quota file format (as we're out of space), it's pretty much straightforward for us to keep soft quota information in the quota file. Keeping it in memory could chew up more memory, but if we're keeping the entire quota database in memory, I'd consider that a deficiency that should be fixed (although it might take some effort to fix it).
It is *not* straightforward to have the user who goes over quota notified immediately if they go over quota on a file system ounted from a remote NFS server, which is why SunOS 4.1.3 didn't do it (and why 5.x doesn't do it either).