The main question is "What effect does a wall have when imposed next to a rotor?" If we picture the rotor as producing a downward stream we then would like to know what the wall does to that stream, and what that means for rotor behavior and pilot workload.
All the following is conjecture, I have no relevant experience or test data to support it, but like a good Grimm's Fairey Tale, here goes: My gut feel is that the wall will behave (if sufficiently close) like an end plate, restricting the outflow, breaking up the recirculation at that azimuth and helping the rotor become somewhat more efficient for that brief moment when it passes the wall. This would produce some extra lift for the power, and also flap the blade upward later in its rotation. For a US rotor rotating CCW fro above, this would seem to want to make a right side wall cause a slight pitch up.
I also believe this effect would be small, easily disturbed by wind or wall roughness, and probably only induced if the rotor were operated within a few feet of the wall (perhaps 5% blade span - 1 foot for a 20 foot rotor- probably far, far closer than safety would dictate).
When we introduce the ground at the bottom of the wall, the wal will restrict the outflow, and probably confuse the recirculation. My guess the effect would be very unsteady, and not benificial at all, costing hover power and controol workload. When compounded with wind, this wall/ground effect would be problematic. Since it disturbs the rotor wash, I believe the effect would be worse for higher disk loading rotors (bigger helicopters).
I am charging my little model helicopter, and will try some wall hovering in a few minutes and report back.