The mirror stems are strong enough for small lights. The best way to do this would be to fabricate a plate that mounts at the base of the mirror, don't mount it to the stem of the mirror itself. The problem is this is way too high for road lighting, and it will blind oncoming cars while not offering good lighting on the road. Off-road bikes put lights high to see out farther but they do not expect oncoming traffic.
Also, the higher the lights the more the light patterns of the fixture will drift as you lean into turns, often putting the light outside of where you need it. The best location is low, using the fender mounts. This will reduce the amount of movement in the beams.