How to query (Group Folder) Usage/Quota on server (cron/munin/shell/SQL, ...)

While I am specifically interested in Group Folders, the question also applies to other users spaces:

How can I find out how much of a Group Folder Quota is used with a shell script?

I know that I can SQL-fu the group folders mount point and then run e.g. a du -hs <mountpoint>, but I assume that the info is somewhere stored in the database as well.

I am not scared of any shell/SQL-stuff.

Background is that I would like a munin-script to alert me when the folder nears “completion” and also work on a visualization of the current state.