We had the same problem so we ended up creating some cron jobs that run on the Enhance control panel. They use the Enhance API to get total disk and per mailbox usage information. It generates a warning email (which we get bcc'd into) that is sent to either the account owner or the mailbox user depending on the email type.
We also run a SmarterMail platform so we were able to adapt the scripts to include the usage information from that using their API as the disk usage is combined with the Enance total to get a total usage.
It all works pretty well. Happy to share the scripts but they are looking at 2 different platforms.
