So this is to stop her from plucking, right? Will she let you pet the bare parts? Maybe you could put the medicine on your fingers and stroke her in the places she plucks and get it on her that way, without Missy actually figuring out what you're up to.

I do that with my basset, who has annual spells where he chews himself raw at the base of his tail but objects to medicine being put on it. So I put the stuff on my hand and pet him and as far as he knows, he's just getting petted. At least, I THINK I'm fooling him ...

It might work with Missy, too!