WEP in AH models 2 separate things: boost from closing the waste gate and boost from injecting a second (or third) fluid into the engine.
So, why not separate all these functions? The waste gate would be fully proportionally controlled, like the propeller pitch, and any fluids would be injected by pressing P.
When the waste gate is closed over a certain amount, the temperature begins to creep upward faster as the waste gate is closed more. Once it is opened, the temperature would begin to drop slightly, then more rapidly as it is successively opened.
The aircraft equipped with nitrous oxide, water/methanol, or some other boosting fluid would have a tank of it, like a fuel tank, and when it's gone, it's gone. You would have to land to refill it.
What do you think?