I would recommend to use postfix together with maildrop instead of postfix with the quota patch. The maildrop solution works also completely out of the database but it needs no patches and you can use postfix from your linux distribution which is easier to keep up to date.
I'am currently writing on such a setup for ISPConfig 3 which is completely SQL based.
The last piece I'm still looking into is controlling domains for Apache through mysql. If you know how that could be done, please let me know.
I'am searching for this too but I did not find any acceptable solution, so ISPConfig 3 will use files in this area.