The only idea I haven't seen mentioned is the flying aircraft carrier. Imagine the dragon lifting a few soldiers with gliders strapped to their backs. At altitude, it releases the archers and they rain death from above. The archers may need to be in 2-man gliders with a pilot/bomber and archer. If the dragon(s) could release the gliders from behind enemy lines, they could cause a lot of havoc, but with the disadvantage that they have to land behind enemy lines or fly over the lines to reach friendly ground. Even dropped from friendly airspace, the gliders would have the benefit of gravity and elevation while shooting/bombing. In this mode, the dragon becomes a glorified elevator which substitutes for a nearby hill/mountain (which may not be present at the battlefield).
If each dragon only had a few gliders to worry about, it could release them at high altitude (where they could start with bombing), let them glide down to some flight floor (where they can snipe with bows), and then pick them up again in mid-air, to bring them back up to the flight ceiling. 160
In between, it could strafe enemy lines and do its normal dragon-thing. The gliders would be a force multiplier for the dragon's flying ability, to potentially devastating effect.
In the same way, dragons could act as long-range siege artillery by lifting battering rams made out of giant logs and stubby glider wings (and, of course, an iron ram head). Since dragons are not necessarily experts at ballistics and aerodynamics, you would ideally want the ram to be guided on the way down (so the dragon can drop it from very high and far away, which provides both safety to the dragon and lots of kinetic energy for a big boom). Some elite pilots could ride the ram down, using basic flight controls, and jump off at the last minute, deploying their personal gliders for a safe landing. If you drop it from high enough, a battering ram will have enough energy to blow through not just the main gate, but pretty much any reinforced structure made of stone, so you could blow a hole through any large part of the outer wall that is easy to aim at. Of course, you still have to deal with the moat, but you don't need a roof on the ram to defend against elevated attacks. Also, the ram should be moving fast enough close to impact that it is nearly impossible to hit with arrows, so the pilots should be mostly safe (except for the fact that they are riding an inert missile!). 160