The more I think about it the more the "aerofoil" theory seems sensible to me. My reason is that lift tends to act on a conventional aerofoil around quarter chord, so in rough terms I'd expect the force towards the door to act from nearer the hinge edge than the catch edge.
As an aside, a TP working for me had a door come open last month in a slightly smaller but similarly configured aircraft to the C150, except in this case the hinge was at the top. The door came open and sat about 20° below the wing, this created neutral to divergent DR with yawing oscillations through ±20° before he got the door closed and brought it under control. Interesting in this case that although you'd have expected immediate roll or pitch effects through asymmetric disturbance to flow around the mainplane, the main effect actually became in yaw.
I then took a similar aircraft out myself and put a force gauge on the door, at the bottom catch, at 50 kn (compared to Vd around 95) I got a 6daN force to hold the door in position with it just very slightly open.
G