It has been done/attempted, and is an automatic loss of certificate! About 15ish years ago there was a guy who attempted to land on (he did land on it) top of the arch, but before he could release his first chute so he could jump off with his second chute the wind pulled him off the north leg like a big slide. Obvioulsy it was his last jump.
Great pics, I was born and raised in St louis area and never did get to fly into KSTL. Spent a lot of time in the other stl metro area airports though!