There is another trick, though it requires changing the init -files more often.
First you need a boulder which will boil on any temperature and causes syndrome when inhaled. Next you need to turn your temperature off for the sake of preventing the boulders to boil too early. After that you must bring the boulders next to the location where you are going to build some stone traps. This can be achieved by stockpiles or dumbing, just be sure that the closest stones (in coordinate wise, not pathfinding wise) are the toxic stones you have produced.
Now, after the stones are hauled to proper place, build some stone fall traps. The dwarves will use your dangerous stones to load the traps. Be sure that there is no more toxic boulders than there are traps, as any stone left in stockpiles will boil when temperature has been turned on.
When the traps are loaded turn the temperature back on, none of the toxin boulders inside the traps should boil as their are objects used in construction. When enemy triggers the trap the stone is released, causing it to boil immediately and releasing the toxic gas. Congratulations, you have just made stone fall traps more deadly!
There are few drawbacks in this: If you turn the temperature on too early or there are more stones than traps the stones will boil, possibly harming your dwarves. Also, if your soldiers are standing near the the traps when the boulders are released there is high chance that they are affected by the syndrome as well, not only the invaders. Also, any wild animal can trigger the trap, so there is high potential of wasting the stones to non-dangerous animal. Lastly, the dwarves will probably try to reload the trap immediately, and as there is no toxic boulders left they will use normal stones. This can be avoided by locking the civilians out, either by really locking the entrance or by burrows.
Now, I haven't actually tested this out yet, but basing on my experience of how the boiling and temperature works I don't see any reason this shouldn't work. Just remember to turn the temperature off before producing the dangerous stones.