Also thought i would mention this,
Trev Fay pointed it out to me at the nationals in Adelaide mentioning it was one of the best solutions for it he has seen. I can't remember which boat was using it, though i think it is a very elegant solution to the problem.
When setting up the tip of the bowsprite you want it to do a couple of things;
1. Be able to get the spinnaker tack as far forward as possible, as the bowsprite is limited in length.
2. Be a low friction setup for the strop to move through
3. Lightweight, since it is at the extremity of the boat hull e.g. effects the rotational inertia of the hull to pitching and turning.
I normally use two sheave boxes, which cost about $20 each and you need to reinforce the tip of bowsprite as you cut most of the material away to fit the sheave boxes.
The idea used on this boat i saw used a turned plastic plug in the pole tip with a hole straight through the tip. The ends of the hole where nicely rounded to avoid chaffing the rope and the plug was made from a nylon or low friction plastic.
I image this could be made to be very light weight if the inside of the plug was hollowed out.