User blog comment:Fliu/Chapter 935: Disaster at Udon/@comment-7614831-20190308070627/@comment-178.21.87.8-20190308081149

As far as I understood, there is no direct connection between Seastone and Haki, however a Devil Fruit user is weakened enough by the Seastone that he can barely exist and not gather enough willpower to manifest Haki. So a non-devil fruit user should be able the use haki without proble even with handcuffs on.