I have a Mamiya 645AFD II connected to Kodak DSC Pro Digital back. I am also looking for a L-plate. So far from both Really Right Stuff and Kirk all I can find is the camera plate for tripod mounting. Another solution is probably look for a wrist support, that attatches one end to the camera tripod thread socket and the other end to the camera strap attachment loop on the side of the camera.
I have the same combination (although AFD not AFD II). I successfully use the Tripod-NII adaptor and then mount an arca plate on the bottom. This provides clearance for the battery on the Kodak when mounting on an Arca style plate.
As regards L brackets for the AFD - I used to use a Kirk L bracket - this works ok so long as you don't want to use mirror lock up when mounted vertically.
The other approach is to use the RRS Universal L bracket with a standard plate on the bottom of the camera. This allows you to mount the camera either left or right side down and hence still use mirror lock up if you need it.
Just a thought: Why would you want an L Bracket when the DCS Pro Back shoots a square image anyway? That's why I don't bother any more. If I were using a wider imager or film occasionally then maybe it would make sense, otherwise it's a waste of time.