This would be somewhat AI friendly. As they build this in their capital city, when they get injured they are warped back there... they should have the probability of losing that injury as they recover and others (with the proper facilities). The idea is that the hospital or healing station be a place to heal your heroes. Obviously I would put this building mid-game or late game.
It would be nice if the AI gets a little boon when it comes to healing injuries. I just thought that this mid-to-late game boon would help out. It could even be upgradable.
Start with a apothecary to healing circle to hospital.
The apothecary, low tech but grant bonus to healing champions in the city. So, when hero is warped back they recover 2 to 3 times more hit-points before they are done being incapacitated. Healing circle raises the healing and offers a small chance to remove 1 injury. The hospital keeps the same healing as before, but increases the chance to remove an injury per turn.
This would also work great for those people that move their injured heros to be governers... You know I used to be an adventurer like you but I got.... I won't finish it for your sanity. They would bring back late game heroes to the fight.
I called this a new feature, because it adds another mechanic to the game and this would have to be included with AI programming and balance to the game. It's simply a lot to ask to make. Even adding new spells can change the current balance.
I still don't know if you can remove 1 injury by mods (is that possible).
Also, it is not a free injury removal. A 25% chance per turn late game to remove 1 injury after building a late game building is not removing an injury for free, it's the possiblity for removing an injury for free. The chance could be made lower. I would say you would have to stay in a city for roughly 4 - 5 turns before an injury is healed, and that is what I am suggesting, late game.