That would actually be one hell of an SBS question IMO.
Till now there seems to be pretty good proof for both sides…
(1. Haki cannot be used on non-living things.
~ Enel was unable to read Luffy's actions when he made himself stupid.
~ Zoro using Haki in Alabasta would be too early in the story to be foreshadowing. He probably developed as a Swordsman.
(2. Haki CAN be used on non-living things and = The Vocie of All Things.
~ Zoro having some uber power which he never used later on, which Luffy doesnt have, and which Rayleigh and Roger DO have, does not seem logical to me.
~ Zoro wouldn't have used the power to read non-living things only once, to never return later on. If it had been a swordsman power that he willingly developed, he would have been able to use it consistently afterwards. Till' this day I believe it was an early Haki outburst, because he had so much will at that point to cut the steel.
~ Luffy could dodge seemingly random water bullets from Hody. I doubt that Hody would willingly know where ALL of those bullets exactly would go to, which Luffy could read in his mind.
I feel that the Enel argument is a really strong one, but I find that it doesn't compare to all the "evidence" / logic i see in the arguments that Haki is also for non-living things. Please correct me if i forgot anything, I will add it and i will probably post it into the Haki thread later, to bring it over there and get rid of it in here :D