The only major advantage I can see of having the traveler on the cabin-top is the increased distance you can swing the boom. The farther aft the traveler, the lesser the distance of travel. Of course, the comparative distance the boom travels will also be predicated upon the length of the traveler itself.
As for stress on the boom, I would think the ultimate anchoring point of the traveler would be approximately 2/3 aft of the mast. Anything forward of that would likely pose a stress point, while aft of that would lessen the stress to some degree.