Two variations on the same theme of exposing oneself to a horrible corrosive in a location which prevents inteference from others:
Stand in a pit of quicklime and get slowly causticed away
Stand in a volcano from which sulphur is mined
Does it have to be slow? Self-immolation is widely regarded as horrific and must take an enormous* strength of will to see through
Does it have to be a done deal? If you want dishonour (and potential redemption) are you not removing a more satisfying story possibility from the other players if your character can be rescued and then instructed by superior to do something undignified to atone (like a Samurai being instructed to shave his head and go to a monastery)?
* and in my mind unnecessary but we are talking here let's pretend and fantasy not real world so my opinion probably counts for even less than usual
Rule Zero: Don't be on fire