I'm not sure what you mean, basically just an inverted foil pan over the ribs with nothing underneath?
The relative humidity level immediately near the ribs might increase marginally from this but I can't imagine it having too big of an effect on the cooking speed of the ribs. In the environment of a WSM the heat is dry, and fairly constant within the cooker. Since the heat is indirect, having a foil cover but no bottom over the ribs wouldn't have a lot of effect imo. It's different than if the heat was direct and having a foil cover would reflect some of that heat back onto the ribs, keeping the immediate cooking area warmer.
The reason that foiling ribs works is because it increases the relative humidity near the ribs to 100% and therefore stops evaporation from the surface of the ribs so more of the heat energy from the cooker goes towards rendering fat and converting collagen connective tissue to gelatin, instead of converting water to steam.
Without a sealed environment like a foil pouch or covered foil pan, the water evaporating from the ribs will just escape from under the grate, and will continue to evaporate using that heat energy just like they were uncovered.