I agree with you that it does not make a difference from whome the organs come from, regardless whether this is a prisoner or someone else.
But I think you should have to have the possibility to opt out, again regardless whether you are in prison or not. At the end of the day it is your body and this is a human right they are taking away from you.
The problem of too little organ donors in some countries could easily be solved with opting out systems. A lot of people are just too lazy to opt out. However also some countries with opt out systems like Sweden have low donor rates.