If the trim pieces are meant to be installed with the headlights in place, having a ball-stud arrangement might be tricky, since it would require enough clearance to pull the trim piece out (toward the headlight) and I'm not sure a ball-stud short enough to allow for that would work very well - at least at the rear of the trim piece where the headlight sits. What about a clip in the back to "hook" onto the fascia, with a ball-stud in the front? Installation would involve pushing the trim piece in slightly more and at an angle, then pulling it forward and snapping the ball-stud into place. Removal would involve prying the ball-stud out, then angling the trim piece toward the headlight, and pushing it in to disengage the clip. I'm not sure the fascia material is tough enough to withstand it, but I'm just thinking aloud here - I know some people (myself included) think the trim piece would be more attractive if the fasteners weren't visible. I don't have my car right now so I can't go out and look at it to check for clearance to see if it's even possible, but maybe someone else could.
Feel free to disregard this message, of course.