In light of keeping surgeweb as clean an interface as possible and keeping some of the existing uncommonly unused features out of surgeweb, that is one of the features I have been pondering whether one wants to leave out of surgeweb. 

Not sure yet though. Your request is only the second request for this feature to be added. Your users can still use user.cgi, or maybe I add it behind a setting that is disabled by default but you can enable in surgeweb if added...

One of the things, that I have long been pondering to make the mailbox-archiving more usable in my eyes, is automatic "archiving" to single folder tree where older mail from multiple folders ends up in a single tree. Possibly with a really simple interface in surgeweb... Don't know yet... opinions on what you would like to see?

eg 

YYYY/MM/Inbox

etc