One idea might be to make a link to the messenger appear like a forum. To show how this might look... I made a sample forum called Insider Talk.
If you click on that link, it takes you into the private messenger. (yourdomain.com/messenger)
Once there, you would simply click "Compose New", type in who you want to receive the message, the subject, and the message itself. You could include multiple people, embed images, etc.
To set this up, you would create a new forum and choose "Redirect" as the type of forum. One of the options would be where do you want it to redirect to. Paste in the address of the Messenger system.
From what you're saying above... you don't feel the PM system would work because it's hidden and buried inside of menus. So the fix to that is to put it in the place you're asking for it to be... in the main forum list itself, which is possible with the existing tools/features.