This is why I suggested making a HIT POINTS section in the early part of chapter 1.
Also I had a thought. Thieves should be able to substitute their Luck modifier for their Stamina modifier for hit point rolls. (Assuming hit points are also considered "luck in combat").
I think it would make more sense to let characters somehow burn luck to soak damage than to substitute luck for stamina. It would give more of a "luck running out" feeling after the character cheats death.
Using Luck for HPs seems like a bad idea since it is a fluctuating stat.
I haven't really given it much thought but a character could be allowed to burn luck when a blow takes him below 0 HP. For each point burnt the character gets their level in HPs. They can only burn enough points to bring themselves back to positive HP.