Well, the variable can be set on a per map basis, so if a mapmaker creates a map that requires a high number of converts/undead he can set the chances of that happening to or close to 100%.
The problem isn't the 255 creatures level cap, that problem must be solved by raising the cap. The problem is that mapmakers find it very difficult to make hard maps where they give the player access to a prison and gems. Players want to have enough gold and those rooms, but they are so powerful that they take the challenge out of every map. If they put a lot of heroes on a map to make it difficult, they will get converted to make the map easy instead.
My proposed change gives the mapmaker a tool to tweak the difficulty in such a way that if the player fights well he keeps his own creatures and might get a few heroes/undead as a bonus, and if he fights poorly he will lose more creatures in the fight than he can get back from the prison afterwards.
I'm not saying it must be impossible to heal creatures in the torture chamber - in fact if you don't heal them they will run out of health before torture works, see the first decision triangle in the flowchart - but that it should not take any sense of reward away from the player. Right now you can get all the creatures you want all the time without even trying. If you have gold use the heal spell, if not use chickens, but every creatures you'll fight will end up in your army. Nobody ever goes: 'Yes, I've managed to get a samurai, cool'. Because it is expected.
And note that bullfrog already put the 'RNG' in torture, they just failed to make it work properly because indefinite healing will negate any effect of torture duration.
And personally I dislike hard caps. What should happen to the creatures in the torture room at this point, should they die instantly, be tortured indefinitely or just refuse to be tortured? And what would be the ingame explanation for that?