in reply to Managing a Personal Email Archive

This doesn't directly answer your question, but have you looked at Pronto!? It is a Gtk/perl email client which uses a DB & DBI for the message store. According to the homepage, it can also import MBOX folders, and export back out to them.

If you can't use the program, perhaps it's DB schema will be useful to you. I used it briefly at a previous job, and it seemed to work fine, although I did end up using another mail program.