I found out a way to create a vortex force field on a curve. It's actually not at all mysterious, I just didn't notice it before. It's an option that isn't available in blender 2.49, so in Blender 2.5x or higher all you have to do is:
1. Add a curve object (Shift-A, Curve - Bezier).
2. Go into the Physics panel and enable Force Field. Make sure the curve is still selected.
3. Change Type: to Vortex and Shape: to Curve(new).
That's it. Now any particle system that is set to be effected by force fields will be effected by the curve.
The red arrows show the curve's effector field:
1. Add a curve object (Shift-A, Curve - Bezier).
2. Go into the Physics panel and enable Force Field. Make sure the curve is still selected.
3. Change Type: to Vortex and Shape: to Curve(new).
That's it. Now any particle system that is set to be effected by force fields will be effected by the curve.
The red arrows show the curve's effector field:
You can have other force fields on a curve too, for instance wind:
This may not seem very exciting, but it's a step in the right direction.