Discourse now supports composing a new (pre-filled) personal message via URL!
To compose a new pre-filled message, modify URL params like this:
http://discourse.example.com/new-message?username=techapj&title=message%20title&body=message%20body
This will open a composer window pre-filled with username, title and body.
Try it out live on try.discourse.org
!
http://try.discourse.org/new-message?username=rudy&title=message%20title&body=message%20body
Want to compose a pre-filled topic instead? See: Compose a new pre-filled topic via URL