I have actually tested this theory several times. In college we used to waterbomb FRAT-RATS (ROTC and the Frats had a running war) off prominent university buildings. We even tried similar stunts at various drill competitions, etc. Take it from me, if the balloon is dropped from five stories or less, it works great. Anything higher than ten stories and it tends to be ripped apart in mid air, I think from wind friction over it's surface....but not sure. I also don't know if you could get enough agent into a balloon or drop enough in the right pattern to insure uniform dispersal. But keep thinkin' Butch! That's what yer good at!
What method were the goblins going to use in Jordon? They had predicted 85,000 deaths IIRC. It was something on the top of skyscrapers. I don't know the mechanics of their plan.
The only efficient way I can come up with is a low altitude sprayer similar to crop dusting. The helicopter couldn't be a tourist thing, it'd hafta be their own.
B/C doesn't fit here.
A helicopter can't do the damage to a building that a jet can do. Different issues entirely.
I hate to say this, but the only effective use for a stolen helicopter would be around 1850'.