I've looked into this a bit myself. Here's the deal - you can't run airbags on top of springs without the bags contributing to the suspension normally. This is because there is a minimum amount of air you need in them.
You could run air bags instead of the coils. This would give you the adjustability. The down side is, the more air pressure, the more lift, the greater the spring rate. So yes, you could set up the bags to run at stock height or 4" over stock, or what ever height you deside based on the attachment cup length. And if you want to raise it to get over an obstical you can. The only possible problem, and maybe the times you want the extra height this won't matter, is you no longer have as good of articulation. Think of it this way, at normal height, you're running say 30psi, which is 300lbs/in spring rate on the bags you pick. Then you pump it up to 60psi, to raise it a couple inches, and now the spring rate is 450lbs/in. It's higher but stiffer. Unfortunately what most would really like is the exact opposite, stiffer at low height for good road manners, and soft at off-road, obstical clearance height, to get more articulation.
If you really want to run on springs, and then want to be able to raise it, and not change the spring rate - use hydrolics. Which is most likely what Dusty was seeing hopping in Cali as bags haven't been able to inflate and deflate fast enough to hope till resently. Pick-up an issue or two of Lowrider (I'm going to catch crap for saying that) - you'll find out a bunch about air bags, hydrolics, and air cylinders. Now, what you want to do is you're not going to replace the springs with hydrolic cylinders and use and accumulator for cushion, like some low riders do. Those are the ones you see bouncing down the street cause there isn't much suspension movement independent of flipp'n the switches. What you want to do, is hook up a hydrolic cylinder (or you could use an air cylinder) to the top spring perch. And what you're basically going to have to figure out how to do is make the spring perch go up and down. I've seen some like this for dancing contests, with hydrolic over springs. You're going to want to use it a bit differently, as you'll want full length springs.
If I was doing a hydrolic over spring set-up, I'd rig up a bell crank so I could lay the cylinders (air or hydrolic) horizontally along the frame - keep them from sticking out the hood. Then I'd mount both the shocks and the springs to the other arm of the bell crank. That way, when you raise the suspension, you're not over extending the shocks. Though some super long travel shocks would work too. I'd put a bump stop (solid most likely) at the ride height I wanted to keep the bell crank from spining, and make it so the load isn't on the hydrolic cylinder at that level. That way any compression in the hydrolic system isn't going to influence your spring rate at that height. The bell crank, if the arms are the right length and pivoted right, will also point the springs more perpendicular to the axle. Remember, and the front arms go down, the axle rotates counter-clock-wise from the driver side, and moves toward the back of the vehicle. The other advantage to using what is basically an adjustable spring mount, with a solid stop, is if anything in the system fails, it's simply ridding on the stops, and performing like a normal vehicle.
It would work really cool as you could run as low as you wanted on the street or as high as you wanted (within your design parameters) off road, and the spring rate would be the same. The question is - is it worth the time and money to put a system like this on your truck? You could do straight air bags for under 2 grand if you already have the rest of the lift parts if you're setting "normal" to X over stock. The mounting cups would be pretty easy to fab, the rest of the system, the controllers, compressors, valves, are all readily available. Fabbing up a hydrolic over spring system, that would be a challenge.
