Well, doing the same poem, and i've been looking around for a clear message, but the only real messages in terms of the inner journey i could get was:
-that our most visceral experiences stick with us, especially when related to human desparation, and the extremes of the human psyhce (i.e what pilinszky finds inedible, the french prisoner eats ) could also relate that our experiences can be shaped by empathy and imagination, as pilinszky alludes to the french prisoners horrific ordeals by describing his physical appearance and his mindset on cattle turnip. Thus he leaves it to the audience to imagine what the prisoner has gone through to cause this.
-Guilt that follows inaction, as the protagonist mereley voyeurs at the event, and doesnt participate in helping the prisoner. (but this is kinda sketchy as Pilinszky shows not so much empathy towards the prisoner as opposed to disgust at his existance.. what a tightass XD)
Just remember, in english, what you get marks on is how well you can present an argument and back it up, not so much on what the true meaning behind it is, just your own meaning + quotes + examples to support it.