An easy workaround is to create a new ammoy type that adds 100% DR to the target, the only problem I still have are insta kills, but then again it is kinda realistic too (if you get some sandbag shot in a critical spot of your head at high speed you might still die from it ;P ) No new overly fancy scripts required.
Scripting?
I managed to code it, here it is (modifications in orange):
Also, if NPC heal rates are low enough, non-lethal shots would be great for preventing reinforcements in guarded towns.