Great! Now that we know what you're after, we can help.
It looks like you're trying to use an
additional save. I think we can get to what you want by making the Death Save more fluid.
Right now, every time you roll a Death Save:
1-9: Mark off a checkbox. Three and you die.
10-19: Nothing happens.
20: Spend a healing surge.
First, rule that every strike for Subdual damage* grants you a cumulative +1 on Death Saves until the end of the encounter or until you spend a healing surge.
Then, we just need to determine how hard subdual attacks need to be.
-1Hit/-1Damage is a pretty minor penalty, even when you assume that you're going to need to hit someone with several subdual attacks to give you a good chance of taking them alive. I'd probably go with -2/-2, or if I was feeling really cranky and wanted to make it hard, -2/HalfDamage.
*It's an interesting idea that
by default Psychic damage is subdual. Maces & other blunt weapons still do plenty of damage though. I'd rule that anyone making a Weapon keyword attack needs to make a special effort to hit less lethally.