Half the time Bartolomeo apparently just covered his skin with it since nobody noticed a thing. Are you saying even in that case haki wouldn't help? If it helped, then erecting a fully separate barrier would make no difference. If it didn't, then I'd say haki performs quite unpredictably. I tend to think it sort of nullifies all fruit powers on contact, thus allowing physical damage to act like it normally would. In this barrier's case, an attack using haki would simply pass through as if it was empty air.