I don't think anyone's done it yet. 180 degree headers are commonly used on American V8s, like a 350 Chevy small block. The point of the headers is to even out and harmonize the firing order of the engine making it sound like a European V8 (Lambo, Ferrari, etc).
For reference:
Now, a V6 doesn't need this, because well, the firing order is already in sync. V4's don't need them (used mostly on bikes). V5's.... let's not go there.
So, what I would like to do to a four-banger, is do the reverse. There are a couple of ideas floating around, but no one wants to do it, because frankly, it would be cheaper to get a 350 small block into that Miata than worry about the headers.
Just thinking out loud is all. If you're interested enough, the next time go to a car meet and ask a Miata owner, it's a quirky ice breaker