I love rosemary and have access to a whole bush of the stuff, so I cook with it quite a bit. Sauteeing works fairly well. I've never tried toasting it, but imagine it might get kind of tough and bitter.
While I love the taste that rosemary adds to a dish, I'm not actually a huge fan of the needles themselves. They can be kind of bitter and they tend to stick between my teeth. Because of this, I often use rosemary like a bay leaf. I add a whole sprig (or more) into whatever I'm cooking, and then pull it out before serving. If I want the whole thing in there, instead of just its essence, I chop it as fine as I can to avoid the teeth-sticking thing.
Not sure how this works into your garnish idea though. Maybe you could chop it fine and sautee it. Or go the other direction and toast whole sprigs. This way, people could either crumble the rosemary over their chicken or remove the garnish if they didn't want it.
This is how I usually see it used too - put the sprig in, and pull it out before it starts to break up too much and get all over the place. Do keep in mind that given enough time / agitation, the needles will start to come off and get all over the place.
I love AOC's cavalo nero dish which features this:
And also this method for Tuscan potatoes, which has you put in several whole sprigs while cooking (which aren't served), and then you sprinkle minced rosemary over the final product.
Sauteeing chopped rosemary makes it bitter--it scorches very easily. I know this from personal experience. The oils in the needles seem to have a low tolerance for direct heat. I can't imagine toasting would work any better, as that's also a dry heat. I think you'll be better off very very finely mincing the needles (till they're like talcum powder) and sprinkling them on, or some other way that adds the rosemary at the very end. Wet heat is fine--in a pot of beans or soup, etc.