I tried to do same way - attaching a light to the clew. I does interfere with night vision, at least at some points of sailing.
My latest solution was to attach
C-light Personal light $11 at west marine, to the top of the sail.
Light runs 14 hrs on two AA batteries. Visible to 2.5 miles. Very small. I have square top sail, so I got longer vertical batten(one, wich goes parrallel to the mast) and Dust taped a light to it. Since light is running 14 hrs, I turn it on when raising a sail prior to night sailing.
I keep a powerful diving waterproof light attached on trampline with line for signaling and making my sail visible when I see approaching boat and I have waterproof headlamp (the one you can wear on the head, sold at camping/hiking shops)...
I found it sufficient for long night trips...
Many sea kayakers, travelling at night, have a nice solutions for compact, waterfroof running lights.
You may ask a question at their sites -
www.watertribe.com discussion board had quite a few discussions about it in the past, you can do a search there.
My 2 cents
Vladimir
Unicorn woodie A-cat