Yes, to an extent, but most "trail lights" has quite a lot of upwards throw, and if you were to aim it low enough, it would create a "hot spot" right in front of your wheel, not allowing you to see much else.

But in any case, how would you point a light on a helmet low enough so that when you look up and down the road it is still low enough to not blind anyone? And, more to the point, my comment was aimed at the people using their light as a weapon against fellow road users, even if my example was using "unthinking" mountainbikers/runners.

People in cars don't run on high beams constantly, and for good reason. I don't see why people on bikes (or runners) should somehow be excempt and them doing what they do, should make it okay to blind other road users. It's not only dangerous, it is outright antisocial in my book.