Generally speaking, I'm uneasy about any reduction from a less-confused concept to a more-confused concept. Free will is a more confused concept than moral significance. Also, I can imagine things changing my perspective on free will that would not also change my perspective on moral significance. For example, if we interpret free will as unsolvability by rivals, then the birth of a superintelligence would cause everyone to lose their free will, but have no effect on anyone's moral significance.