Note that Gmail automatically saves your draft as you type it. So while this will offer some protection for the message while it's in transit from Google's server to your destination, your unencrypted message draft will still be sent to Google's servers (and given Google Apps' distributed architecture, I'm not sure you can determine where that unencrypted copy could end up or when it'd be erased).

Perhaps a way around that could be to enhance a text input field so that only the final encrypted message is written to it, so Google's app does not see the plain text.