The ledger would lay flat on the 5:12 roof, and there would be flashing for
the water from the asphalt roof to run down over the flashing and onto the less sloped polycarbonate roof.
I have not thought of your suggestion, but I have thought of something similar. Not doing a ledger but have the patio roof joists just sit on top of the top plate/house band and tying it in there.
I appreciate your thoughts.