We block the draft from going up to Gmail's servers. Gmail only sees the encrypted version of the body.
In a comment on encrypting gtalk, I explained how this can be done: https://news.ycombinator.com/item?id=5858375
Like we said, this is a starting point. It's open source. As with any type of security product that has a hope of being good, try and break it and let's fix it together :)
I believe that Google does releases of the entire application at once so this method should detect when a roll-out has occured.