What link have you got? Got any spare outputs? If not, you could probably get a little CAN expander with some inputs and outputs (Maybe a PDM).
Use it to monitor meth tank level, turn the pump on, drive the injector/solenoid.
I would imagine you get a water/meth specific pump that's got its own built in reg.
From the internet -
Water/methanol must be injected above 60 psi to properly atomize into the engine. Pressures lower than that will reduce the cooling effects, since the larger droplets exiting the low-pressure pump reduce total surface area, and will cause little to no gain in performance.
You could make a table that injects vs throttle position and manifold pressure.
Then do all that tricky stuff to pull fuel out and change timing when it's active etc