Did you have a look at http://dbmail.org/ ? And/or http://aox.org/ (Archiveopteryx) ?
I've been thinking of using one of their schemas as a base for doing something similiar for an email client/interactive mail archive type thing (think: hyperkitty/pipermail).
Also notmuch has done some work for mapping email to xapian for search/tagging/indexing -- could also be a good source of inspiration I think.