I think I can stick with XFS quotas for a while. Not so nice solution but still usable.
If you currently use XFS group quotas for domain-based quotas, you can make this work with Dovecot using a patched filesystem backend. It will enable reporting of group filesystem quotas (in addition to using inode filesystem quotas for message number quotas).
I have been using this patch in a production environment (Linux with ext3) since June, and it has been working perfectly for me. I last updated this patch for rc7, and it probably does not apply cleanly to recent rc versions. I'll update the patch again once 1.0 comes out, but doubt I'll have time to do it before then. You can probably use the patched rc7 version of quota-fs.c with rc15 without problems. I attached both the patch and the patched quota-fs.c files to the email previously.
http://www.dovecot.org/list/dovecot/2006-August/015662.html
Scott