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