Gary,
Again many thanks for the advice. I see what you mean re: the bulbous end of the shackle. I wonder if a larger diameter tether line would help to better slide off the shackle - I bet you have already tried that though.
What have you found to be the best place to attach the shackle - I know the ankle didn`t make your short list!
As I understand you, the shackle stays on the sailor along with a pull-cord to easily release/open the shackle pin.
Since the load rating of that shackle will far excede what is required as a safey tether, perhaps partially grinding away, then re-polishing the bulbous tip would also help the release...(/jokes off /off /off)...down boy! /jokes off dammit!...This could also be done with a larger snap shackle so as to provide a good margin of left-over metal after the grinding/polishing process.
Dave