I think the oxygen added is negligible because the krausen and CO2 are going to blanket the fermenting beer and protect it. It's all about fermentor geometry and hydrostatic pressure. Increased CO2 concentrations inhibit ester production, and the deeper the fermentor is, the more CO2 stays in solution because of the hydrostatic pressure. This has been well documented in commercial trials and is published in Brewing With Wheat if I remember correctly, though I've heard it taught in Germany as well when I was over there. You can get a similar ester profile in a CCV with a low enough height-to-width ratio, but most CCVs are just a lot taller than open fermentors. Then the geometry affects convection currents, which also affect CO2 in solution. On a homebrew scale, I see no reason why not covering a carboy or bucket would do anything. The pressure under the airlock is probably negligible to the CO2 levels in solution. A super-shallow pan for a fermentor? I could see that maybe having an affect, but can't say for myself.