What would happen if I mounted a system dumping hydrogen (sound velocity 1310 m/s) at the leading edges of an aircraft flying at i.e. mach 2 (ignore all practicality issues)? What would happen at the hydrogen/air interface? Would the aircraft experience wave drag?
Consider that, without any hydrogen, the surface of the wing is coated in aluminum, a material with a sound velocity of 6,300 m/s. It's not the speed of sound on the surface of the wing that matters. It's the speed of sound in the medium it's passing through.
You are proposing a wing coated in hydrogen, but even if that could be made, it would still have to move the surrounding air out of the way. It is the "moving the air out of the way" that creates the mach effects.
Assuming sufficient flow to engulf the entire airfoil and that the free flow residency time is much shorter than the diffusion time (to assume the injected gas leaves downstream before it can mix, this assumption is likely wrong!) I see two issues:
The airfoil's shape is effectively changed, because the inbound flow now sees the airfoil surrounded by a sheath of hydrogen, which pushes the forward stagnation point forward, to say nothing of the rest of the foil.
The local $Re$ number in the boundary layer is changed due to the different kinematic viscosity of hydrogen. I can't run the math right now, but that will undoubtedly affect the location of the transition point, which will be good or bad depending on the airfoil.
If your goal is to reduce the effective Mach number, then it may work if the first assumptions hold (which is doubtful), but at the cost of a slew of other issues.