According the first poll results, I've begun to work on the next plugin that will let users define server-side messages filters.
A first development release is available into the repository (here) or with the latest development tarball (here). Currently, it only provides a raw editor and a managesieve client. The result is that you can write your filters inside Modoboa (if you're familiar with sieve syntax) and upload them to your server.
Soon, I'll add a more simple editor (with an intuitive UI).
It has been tested successfully with Dovecot 1.2.9.
Hope you'll enjoy it!