Hmm, I didn't realize that building destroyers would take out pressure plates (how did I not know that?).
Sorry, maybe I phrased that awkwardly; the Dragon didn't destroy the pressure plate, just the floodgate that closed. It didn't recalculate it's path to try to go through the new open gate, just kept going straight through the closed one.
Ah. Got it. Try a bridge instead then.
The trick there is that floodgates fully toggle, but I don't think the bridges do - I don't believe you can hook a lever to a raised bridge and a lowered bridge and have the one lower when the other raises. Might need to experiment a bit there.
How did you get so much carnage out of one dragon?
Keep in mind that if the dragon gets one fire breath off on a bunch of approaching dwarves, particularly on grass, you're thoroughly boned. The dwarves will run around on fire, the remaining dwarves will rush the dragon and run into the fire. If you don't have everything forbidden on drop, then the laborers will come out to pick up burning clothing and they'll catch on fire. You can't control anything because of the smoke - it's just a mountain of chaos and carnage.
In a previous version (I think my first 3D fortress) I had a dragon show up just before a goblin siege arrived and they went at it. The dragon took out 4 or 5 squads of gobbos and some trolls - I don't think any of the gobbos ever reached the dragon not on fire. It was very awesome.