BRP0 has a half-page 'Spot Rule' for "Cold, Exposure, Hunger, and Thirst". Heat/cold increase FP loss rate; similarly after 1 or 2 days without water/food. After CON/2 days, any physical action requires a Stamina roll to avoid 1hp damage. After CON days all actions are Difficult and wounds heal at half rate. Each day on -ve FP lose d6hp. Maybe even lose SIZ! But at least you can stay alive for a day on 0hp.
If that's not detailed enough for your purposes, it might be good to see what 'expanded Spot' (Patch?) Rule you do come up with.