Plasma 'stealth' is technically possible, but not operationally feasible. This idea have been thrown carelessly around by the Russians but so far have never gone beyond the printed paper stage. Plasma emit its own signatures, which can be detected by a different type of sensor. You are using a substance whose properties can influence or even negate the behavior of another substance, which in this case are EM waves, but then emits its own properties. The appropriate analogy here is water that can extinguish fire but give off steam. Radar low observable thru manipulation of the EM wave's behavior changes nothing of the EM wave's properties and create no additional signatures.