Thanks for the responses, even if some are somewhat conflicting information :)
I think I can conclude that most "typical" DP's can kneel down by deflating the air bags, but not actually level with them. Personally, I think it would be nice to be able to discretely level (no jacks down) when stopped for a few hours on a street or parking lot, even if you cannot achieve perfect level.
I am also not a fan of lifting wheels when leveling with jacks, especially the rear (parking brakes anyone?). I have seen some desert camping where they have used blocks to do extreme leveling and end up with front wheels WAY off the ground. It is scary to look at.
A custom control system, as IJ2634 describes is tempting. But I think you need to be aware of how you are loading some parts by unloading others. It would be easy to tweek the frame.