I had the idea of awarding an extra victory per fight if the heroes don't kill anyone but since the vanilla "Knocking Creatures Out" doesn't carry any risk this wouldn't work.
I'm thinking if you want to do non-lethal damage you take a double bane on the attack and you have to do the last point of damage as non-lethal so for example if a creature has 10 hit points you can do 9 points of damage normally and then if you do 1 as non-lethal they're knocked out.
I'm very much thinking out loud here so any feedback or suggestion would be much appreciated.
Thank you!