shadow pulsar
When I've seen Stanley holding on for dear life in levels like the old pit, I've shot him multiple times in the hands and he never lets go despite having a massive bullet hole in his hand. His ulnar nerve, radial nerve and median nerve are most likely severed from just one bullet in the centre of the hand, so ideally... He should have his hand go limp and have him fall, rendering him unable to grab things after. (Not implying you should program all 3 nerves onto the hand, that'd be a pain to code and isn't entirely necessary) My request is basically for his hands to be unable to grab after being shot in a certain point, similar to when you shoot the tetraplegia and the body goes limp. Except in this case, it's just his hand. It'll be a new way to experience the "Moment at gun point" thing you mentioned when developing Standbox. Stanley hangs for dear life, you shoot his hands and he plumets to his doom. (or just shoot him in the head, but that's no fun)
As another clarification, not suggesting that he should let go after getting shot in non vital areas. If anything, the pain should make him hold on tighter. (Not literally, no need to program grip strength into the game-) Area's like the Heart, Tetraplegia and Maybe Jugular should be able to disable his holding on function too. (Could be temporary, but may as well be permanent since Stanley will die anyway.) As for the femoral, well... that's his legs. Should be able to have that as an exception.
Another feature that may be cool for this topic too, is Stanley letting go when fainting from Anaemia. (If he doesn't do that already, I tested the things above but not that)
Obviously all things I mentioned is just an idea, and you as the dev can choose what goes and what doesn't with this idea. What you wanna code and how you code it is always up to you. Warp the idea as you see fit, thanks for reading. 
I'll also simplify what I'm suggesting if need be, just ask I'd be happy too!
strong fossil
stiff owl