Dragon pillbox sounds quite fun actually, but you're going to need something to stop it from getting shot. Most surefire way would be to surround the fortifications with floodgates, which are linked to a lever. You flick the lever, the pillbox is exposed and the dragon can aggro and shoot fire. Flick it again, and bowgoblins can't see or shoot it.
I don't have much dealings with fire so I don't know for certain how to handle the issues of burning equipment and endangering your dwarves. Perhaps, place a water-tank up above the field where the burning is going to happen, perhaps right on top of the pillbox, then when you flip a switch it blasts out and sweeps everything into an underground area. This accomplishes two things: hopefully the items won't be on fire anymore, and secondly, the items will be pushed inside the fort where dwarves can safely gather them, at least some of the time. Of course you make the underground area big enough for the water to evaporate, or drain it.
I would think such a pillbox should be well away from anywhere dwarves would regularly travel, but to herd invaders and the like past it without micromangement you'll also need to herd caravans past it, and therein is the risk of the dragon trying to burn a goblin and setting a wagon full of goods on fire, which then go into your depot and.. problems occur. That's what draw-bridges and flush switches are good for when dealing with fire I suppose?