Overland Travel System. The gist is that a character would take hp damage over time while stumbling around in the outback, getting coated in leeches, suffering malaria, and stubbing all ten of their toes. He's got seven options on his table. I'll offer and eighth.
Hit Points have always been abstract and non-affecting. A character with 100 hp is just as able as a character with 1 hp. It's just that a player with a character with 1 hp is a lot more frightened to put the character in harm’s way. The RPG Savage Worlds has a mechanism that I have found very useful called the Fatigue Track.
The Fatigue Track has five states: Normal, Fatigued, Exhausted, Incapacitated, and Death. This track is separate from a D&D style hit point system. Each point on the track has an effect on the character.
Normal - Character is peachy keen.
Fatigued - Character is at -1 to all actions (would equate to around a -5 for all actions based on a d20.)
Exhausted - Character is at -1 to all actions (would equate to around a -10 for all actions based on a d20.)
Incapacitated - May be conscious, but is pretty well useless and can't move.
Death - Self-explanatory.
Various 'Hazards' kick a character down the track, including Bumps & Bruises, Cold, Disease & Poison, Drowning, Fire, Heat, Hunger, Radiation, Sleep, and Thirst. Each type of Hazard has it's own specifics and usually requires a Vigor (Constitution) check to avoid. Rest and other types of healing kick a character back up to the top. Grab Savage Worlds for specifics (just $10 bucks.)
I think something like the Fatigue Track and Hazard system would be far more effective at replicating the pains of overland travel that hp reduction. Of course, I haven't worked out the specifics or play-tested, so it's just vague conjecture for now. :)