Cooper's jump
If Cooper walked down to the lower part of the stairs and pulled his ripcord while facing forward he would have had a surprisingly nice opening and would not have to deal with the considerable problem of trying to make a stable freefall with no good visual horizon or azimuth references. Look at how gentle these "squidding" openings are (static line jumps from a SAT 727 flying over Thailand for the CIA) . Site wont let me post URLs until I have ten posts. Go to Google and search CIA 727 jumps. The S/L jumps start at about 2:10 in the Flying Men and their Flying Machines video.
I've made over 1000 jumps and skydived from a DC 9-21 (over the decommissioned Chanute AFB) in 2005. Despite what the FBI said, a ventral exit jet freefall jump isn't particularly difficult. Avoiding a freefall spin at night with clouds obscuring your horizon is difficult. The solution? Pull off the stairs thus emulating the CIA 727 static line jumps proven to work in 1970. If DB Cooper served in SE Asia I'll bet he knew about these 727 jumps. Whispers travel fast among jumpers when a new aircraft is jumped. Skydivers knew about Google's STC for Gulfstream V jet jumps long before it became public.
Mark