Make sure that the rack that faces your back has very blunt edges. I have seen this done by a local rider about 30 years ago. He used a cut down scoop shovel that had a hinge that was welded onto the rear facing rack. When folded, the device nested over the tubular rear facing rack and was useable in the folded position. When deployed/unfolded it covered most of the passenger seat and provided an additional large rack area. It was somewhat crude but effective.
The catch to this was when he got rear ended by a drunk driver while stopped at a light. The deployed forward facing racks sharp leading edge cut deeply into his lower back/buttocks. While he survived this, trust me, don't design anything that has sharp edges or that can impale you.
working on the back rest to fit my rack. does anyone have any input as to what height and angle the back rest should be? i am making the back rest slide with, about two inches of up and down movement with the bolts that hold the back pad on. has anyone had a passenger on for a day, what do they think? i am also trying to make it slide front to rear about two inches. any input would be appreciated. thanks dale