Gave up trying to put a tail light on a helmet because it was impossible (for me) to aim it properly. The current front light setup I'm running also makes a helmet light superfluous. I did try for a while with a 700 lumen light - it just didn't make any visible difference. Not to say someone else might feel differently with their particular light setup. I do have a helmet with a head light installation on it - but it gets used mostly as a work light at this point.

But I like reflective tape on a helmet - does that count?