I can't point to BMW direction for using air above the oil as part of the suspension...I suspect that for 98% of average riders, it doesn't really make a difference. However, my quote of Tom Cutter was specific to our BMW forks and he is a racer and knows a bit or two about setting up a bike. The other parts of that discussion by Tom was about the lengths that some people go to measuring the oil volume such that the amount of air in the forks for their setup. So, it is a very real thing...maybe not for the average rider.
As for putting an air valve on top of the fork, I don't think I can find it, but I do recall reading about doing that sometime in the past 15 years. That's all I can remember...it's been discussed before...not here but on the Airhead email list.