I know, it's kind of wacky, eh?
I have done PHP cron jobs before - to run MySQL queries on a schedule, to grab content from other sites, etc. You can use wget or lynx as the cron job to call the script.
I gues I'm just going to do a cron job that has PHP check the size of the mailbox, and if it's greater, send a notification, and if it's the same or less, just note the new size.
Unless anyone out there has a better idea?