I think it can work nice with D20
You might consider Gestalt for more options, but that isn't really necessary.

What should be included are the rules Armor as Damage Conversion from unearthed arcana.
If the armor transforms some of the damage to non lethal the single player is much more likely to survive even lost battles.

Of course that requires enemies who are more likely to take you prisoner than to ..say eat you.

And you might need to think about various escape scenarios from dungeon cells..slave camps etc.