Good point about lower frequencies with multiple injectors.
I don't see a need to make the meth injection valve match the fuel injectors though, if you've got a circuit that can watch the injector pulses, subtract a couple/few milliseconds, and pulse the meth injector with with whatever duration is left over. That would give no meth flow and low IDCs, with meth flow gradually ramping up with air/fuel flow from there.
You could make it microprocessor-controlled and table-driven if you want to make it fancy (RPM on one axis, fuel IPW on the other axis, and meth IPW in the cells).
Does the valve above work well at lower rates? Down to 10hz for example?
Can be put into a housing with an integrated nozzle and dimensions resembling fuel injectors? What would it cost for a set of four?
Combing those with 2-injector-per-port TGV housings, and a water/meth-compatible boost-referenced fuel pressure regulator, and it's halfway to a sweet kit.
Expensive, but sweet.