Here's what I would do: First project your point onto the plane of the disk. Then check if the distance from the projected point to the center of the disk is less than the disk's radius. If if is, voila, closest point. If it is not, scale this distance to the radius of the disk, which will give you the point on it's perimeter that is closest to your original point.
Actually, I think this should be pretty close to what you have do with your rectangle light.