Hey, two suggestions here
1. Include a shortcode so we can build our own notification bubble / button outside of the admin bar
2. Extend the shortcode [pm_user] so it not only allows to prepopulate the subject, but also the content of the message - this could be useful for message templates. For instance we are building a user to user rental system and it would come in very handy if we could pass the shopping cart on to the message body.