Might be too powerful, but what about.
Second Wind [9th level]: The Unbreakable can make a DC 20 Heal check up to once each day for every two points of con bonus (this takes a full minute of concentration and provokes an attack of opportunity) to heal themselves as the spell 'heal' by snapping bones into place, removing foriegn objects, and putting organs back into their proper place. This effect does not cure mental illnesses but does eliminate any physics injuries. ie it would fix: ability damage, blinded, deafened, diseased, exhausted, fatigued, nauseated, sickened, stunned, and poisoned. It also cures 20 hit points of damage per level in the prc, to a maximum of 200 points at 10th level.
Like I said, would take some work but it would effectively look like the guy was opening up his side and syphoning out any injected poison, coughing out any inhaled poison, shaveing off ruined parts of his body from disease (like removing a cancer), pop eyes back in and fix them, reset the body from being exhausted or fatigued, etc etc. It might just be easier to give the list than say heal.. dont know.. sorry to ramble
Instead of mettle you could just give a bonus to saves vs 'list of effects' equal to his levels in the prestige class. At level 10 a +10 vs poison is helpful, but does not make you immune. But only the strongest of poisons will phase him most of the time.