Get yourself a good water-proof flashlight, tape it to your mast tip pointing down onto your sail; that's how we used to do it. I believe state boating laws will require you to be illuminated. I don't think they really care if you can see where you're going.
Good call, I was thinking about looking for some glow in the dark tape to put on my sale and reflective tape just to make me look cool. I know the lake so well I don't think I have to worry about being able to see. Not so sure about not being able to read the wind gust coming across the water at me and judging ramps. Did you Biff it a lot cuz it was dark?
I know this is no help for windsurfing on water at night, but I do quite a bit of night sailing in parking lots. Most of the lots that are big enough to land sail with my various 4 wheeled boards are also lighted at night. I usually use parking lots that are 200 ft x 200 ft or larger (public schools, big box churches, university lots when students are gone). Its good sailing handling practice for jibes, old rig gear works just fine, there might be several suitable lots nearby. I wear protective gear such as a motorcycle helmt plus elbow & knee pads. The wheels are 4" diameter or larger to avoid being stopped by unseen pebbles at slow speeds.