I've recently added YubiKey support to both webmail clients since there's no cost to support it.
It's very early stage right now since we wanted to get something working to work with it ourselves. One drawback is that it needs to be activated in both webmail clients manually. We'll need to either add a hook to prohibit login to the other webmail client if one has YubiKey enabled, or sync between the two somehow. We also want to add an option to the account control center to turn off normal IMAP/POP3 access from locations other than webmail since standard IMAP .
Seth Mattinen, Roller Network LLC