Strike my last email. After quite a bit more time I realized that for Maildir quota support, you need to have a completely different setup for the quota field! namely you need varchar(20), and then slightly different configurations for the mysql db.
Well, if anyone is curious how to set up with maildrop and courier-imap support, go to http://www.xs4all.nl/~jaspersl/quota/
and you will find an explanation for setup which works brilliantly.