from my recollection, as you said, the HT mast ball is not a continuous threaded rod- but there is a spacer I believe between the inside top of the beam and the bottom. This makes it an absolute PITA to replace the little nipple on the front beam the mast usually sits on- which was bent on the 2 HTs that I worked on. I am not sure how to mate a nacra style ball-accepting mast base onto the HT's nipple- it wouldn't work. Guess the only options would be to either replace the mast base with an HT base or somehow mount a ball onto the mast base of the beam. I think figuring out the ball would work better since I doubt the mast base would be compatible without a fair amount of modification.
I would get a SS threaded rod and re-create a nacra style mount all the way to the dolphin striker with nuts and all.
hope this helps... but is unlikely to!