By interupting your pulse from the stator, current will take the path of least resistance and go from the stator to the frame. There should be NO voltage coming from the ECU to the stator so the ECU will not be grounded unless the ECU is faulty. The current will not make it to the ECU so it cannot monitor voltage to the coil and should get an error. Using ground to switch states either way is common inside your electrical components in your home and shouldn't be a big concern unless you are grounding a wire that is hot from the ECU. If you are concerned about grounding this wire you should also be worried that the pulsing voltage of the stator could fry your ECU as the voltage coming off it is pretty high.