Anyone else who tried to harness train their cat?

We do have a fenced yard, and our cat cannot escape. It gives her a nice area in which to wander, sniff everything, and enjoy the sun and all the outdoor smells wafting in on the breeze. I have read about the correct way to harness train: very slowly, over a few weeks, acclimate the cat to the harness and reinforce with treats. But our kitty, aside from already having an outdoor space, is a hard-headed tortoiseshell and her "tortitude" would, I'm fairly sure, short-circuit any harness training fairly quickly.