Yup, know the 18 traveler very well. Try the suggestion of replacing the slug with another ball.

I had considered the idea of putting a traveler track on top of the crossbeam as well, but never got around to it. I think it would work decently, although the curve of the beam may work against the free action as well - as you travel out there will be a sheeting action as the distance between the beam and boom will grow, more so than just the usual straight track versus curved arc the mainsheet would really like to take. What the heck! Give it a try and let us know how it works out!