My tank has a glass cover as well, which may also prevent some out-gassing. A lot of people talk about the splashing noise of the Biowheel-type HOB filters and my water level is so high that I almost never hear it, so I may well be beating the odds by keeping a high water level and a reasonably tight lid on top.
I'd be interested in seeing some testing on the subject, too -- I'm happy with the co2 levels I seem to be getting with my biowheels in place, but I'd like to know the science behind the results I'm seeing. I'd also be interested in seeing test results on various methods of getting the gas into the tank to begin with. A lot of folks new to CO2 use one of the more passive methods to get the CO2 into the tank (airstone, ladder, spiral ladder, bell, glass diffuser) instead of a reactor and it would be interesting to see a test of which is most efficient, all other things being equal.