I think I've figured out what's "wrong" with my horse.
went from being a laid back dope on a rope to Dennis the menace.
He gets 24/7 turnout with unlimited hay, no grain, and he's worked 5 or 6 days a week, but his playfulness remains.
So you want the "laid back dope on a rope" back is what I gather?
I guess I personally don't find "playfulness" to be "wrong" for a horse.
You never said.... how old is he?
You say that he's worked, but what does that involve? Riding? Ground work?
He chews my clothes, licks my hair, ...
This first bit, as already mentioned, it not acceptable. Chewing and licking can lead to biting, so it is good that you do not allow him to get away with it.
...knocks over muck buckets, and is always so, so interested in whatever I'm holding.
But I fail to see what's wrong with the second bit? He's curious. He interested. I'd rather have a horse with some personality, than one that doesn't. *shrug*
My Shotgun is like this. I can't leave ANYTHING within his reach, or it is in his mouth or he is playing with it. I even once witnessed him picking up a cat
by the scruf of its neck with his teeth
(in a nice way; wasn't hurting the cat) ...... I would have never even believe a horse to do this had I not seen it with my own eyes, and the BO saw it with his own eyes right beside me! (We had a good laugh about that, and cat seemed to quite enjoy the attention.)
But my horses always seem to have tons of personality, so I guess I don't mind playfulness at all.