Only living organisms can use it though

Who said?

It was stated by Rayleigh if I'm not mistaken when he was teaching Luffy, or it was a different moment. Thing is, even if it wasn't stated by him, haki in general is basically spiritual energy, and non-living things don't have a spirit/soul.

He doesn’t know it as far as we’re aware, but if he were to learn it, I think it would be pretty strong since he says his spirit is stronger than a normal person’s

Also the fact he stayed sane after 80 years of isolation is a pretty insane feat for his willpower

He has the capability to learn Armament Hardening and Observation Haki. Brook died, yes, but remember, his Devil Fruit is the Revive-Revive Fruit which grants him a second chance at life and the ability to manipulate his own soul. He isn’t undead, a revenant, or a zombie, he’s a fully revived human.

Anything living has the capacity to learn it

@Atlas D. Aeradon Now this is the answer I needed to hear

@Charys Da 2nd glad I could help.

Most definitely, his own haki can even look cooler bc he's manipulating his own soul

Idle transfiguration haki coming???