What about just modifying GMVault to do this?
Re:Automatically encrypt your Gmail account
by etherael on Tuesday June 25, 2013 @10:48PM
(#5943847)
I had a brief look at the GMVault source to see if there were any obvious msg constructing bits, but nothing is jumping out at me, do you happen to know where the message construction is done here?
That said, I think this is worth doing, so it's currently in progress. I just don't have a ton of time at the moment, pull requests eagerly accepted though. :)