I went through this same thought process a few years ago. What interested me
was the ability to have lights that would be visible, at the same time, both
down low for boats and up high for ships.

I have seen them used on a few training ships and tall ships. Usually there is
just one light mounted on a strut a few inches forward of the mainmast, one or
two meters apart, which would give a blind spot for a few degrees aft. This
might seem like a concern, except that the stern light is still visible directly
aft, so it's not like the boat has no aft lights showing at all. And as for
sail vs power, ANY boat behind you is a give-way boat, so all that matters is
that they see you.

In my case I could't readily come up with a good design for struts to mount the
lights away from the mast a few inches, so that's where I left it. But I still
think it's a good idea, and may get back to it some day.

Steve Christensen