A magnetic field such as the sensor's will go right through, unaffected, any none metal material. Any metal particles will simply conduct the field as well. There would have to be a large amount of metal, such as the size of the sensor itself, and all on one side to distort the field pattern. The weakest part is the wire lead. You may want to provide additional support to it to prevent constant flexing due to vibration. If the ring gear teeth were completely filled in with metal particles that would prevent the necessary fluctuation of the field that is required to generate trigger signals.