I believe the air line comes out the bottom of the spring perch but on top of the axle, so you just run it down the axle and up to the chassis with a brake line or breather tube. I ran a set on another 80 once and used the Air Lift pressure sensor valve hooked up to an air compressor mounted where the subtank would normally sit (on top of the spare). This kit automatically keeps the bags at 5 PSI minimum and extends the bags' warranty as well. No human intervention required (except turning on the truck to give power to the compressor and valve). At the minimum PSI setting I detected absolutely no spring rate change. As long as you match load to air pressure (i.e. keep the truck level as you load it up), in theory you should not have any spring rate change as the bags are simply "assisting" the coils in holding up the increased cargo weight.
BTW, I don't believe there is any rule that says you couldn't run the air line out the top of the coil. I think they route it that way in their instructions because it is easier for most people to drill the bottom perch, or there is usually a hole big enough already present (or if not, there is at least a small hole already there, like on the 80, that can serve as a pilot hole - BTW, on my 80 I didn't bother enlarging the hole, just wrapped the air line in successive wraps of military duct tape to prevent abrasion and ran it through the hole - no problems for as long as I owned the truck). For those who want to take the trouble, I don't see why you couldn't drill the top perch and run it out that way.