There would be one piece of client software, which would be an email client and a jabber client. It would be able to load PBEM UI files -- these would be small pieces of code which could take the game state (as received from the referee), format an email message to represent it, receive an email reply, and parse the move out.
(This doesn't have to be JS code. Perl or Python would be easier on everybody's nerves.)
Issue: authenticating the player over email. There may be a JEP which deals with this. Alternatively, you could entrust your Jabber password to the gateway. (If we run the gateway on volity.net, this is no big deal. We can validate the email any way we please, for volity.net addresses.)
Alternatively, have people play with a JID of real%address.com@pbem.volity.net. This loses the ability to share games and game history with your real address, but it's easy.