I reckon it's just supporting the weight of the sailor -- a bit like those somersault rigs you get in gymnasiums when you're learning to forward roll on the trampoline.
So rig up a mast with lines going from the top and bottom of it to the clew (to hold the boom steady), rig up a winch/hoist/strong line from the roof with a spreader bar which then attaches to the sides of your harness, and you're away.
Coincidentally I was daydreaming about just such a rig just a week ago