I am trying to find a good camera bag/messenger bag combo, and I think I have cobbled together one with an Imago plus a Snapr35.
If you are not familiar with a Snapr35, check out their page:
Amazon.com: Black Rapid SnapR 35 Midsize Bag and Strap: Camera & Photo
Basically it is a camera bag with the camera attached to the strap of the bag. I like this system as it allows the camera to be securely attached to the bag, but still easily removed via a snap.
I have jerry-rigged the attachment to fit onto the Imago shoulder strap. The Snapr35 fits into the Imago pretty well, and I have tied it to one of the o-rings inside the bag. The Snapr35 opens from the top, so its very easy to slide the camera in and out. The camera can be placed inside the Snapr35 while still attached to the shoulder strap. I have attached a few images to show this.