After a few months of testing I've decided to add a back zipper flap to the standard design of The Avenue B backpack pannier.  The flap is similar to the one on The Woodward - two quick zips and the backpack straps are secured.  The main difference is that on this bag, there's no spot available on the front of the bag to stow the flap when in backpack mode, so I added velcro to the underside of the backpack straps so the flap can secure back underneath the straps when not in use.

backpack pannier prototype

I'll include this new feature with the next three orders at no extra cost while I figure out how to price out this change.