I believe tea tree oil to be too aromatic and strong (toxic) for chickens.
What I do for scaly leg is to dip the leg and foot in mineral oil for two weeks daily (toward the end I migrate to every other day). Don't forget to dust for mites - don't get it in their face, nose, and mouth. I have read it can attack the face, too.
I swear by tea tree oil for humans (mosquito bites, spider bites, burns, etc). It's worked wonders for my family for all of those things and more, but have never used it on my chickens! To be honest, I never thought about it for that purpose.
(then again, I've never had an issue with scaly legs before either) Given one of the links provided with basic instructions for it? I'd give it a try! In fact, I've bookmarked it for the if and/or when I may need it!