I think this would be a great mechanic. Something to bring the infection more meaning. Players couldn't just kill themselves and call it good with a little death penalty, but instead have to worry whether or not they will cause danger to their friends. Possibly, this could have a chance to turn them into a screamer and alert other zombies.
Tricky but doable I think. Would need the code in the 0-Score, zombie makeup item (yea, an old A15 thing no one uses anymore), change player faction to undead, redo buffs to force player to eat a player death item, remove stamina requirements etc etc. lots of changes but all doable.
This sounds like a neat idea to add to it. if they just turn into a zed, easy to take them out... but if they immediately scream, too late!
Adding some thoughts, though I'm not good enough to make a mod like this happen...
Having infection go to 100% is a natural target.. but having it get "near" 100% and the zombie effect takes over might be a neat effect. Meaning: you will not know when you turn, just "OMG. its over, I just turned!" say ..... err over 50% infection this effect could happen so it could be "now" or anytime as long as it was over 50%. The infection (in vanilla game) isn't really treated like "zombie virus" but more of "bacterial infection"... I feel if you can randomly" drop and turn into a zed after not treating the infection, then the counter/stat for "you're turning into a zed" should almost also be hidden and you don't know when its going to happen ( other than "you've been scratched/infected and its over 50%"), and only if you cure it quickly may you get away with not transforming into a zed. Then if you're infected and near 50% .. you'd have to tell your friends: "um, yeah, I may turn at any moment" and they'll have to all just be super careful for hours...
another thought: after hitting certain infection levels, make taking antibiotics/honey/etc "less effective" in dropping infection so its harder to just cure yourself later in the game, and almost certain early-mid game you're going to turn or die once infected.