I use the toothpick trick on furniture repairs frequently, but on thin fiberglass I would not. The fiberglass just does not have enough thickness (in my opinion) for a good hold by compressing soft wood.
When customers ask what my technique is in making a repair to something (in which the toothpick approach works). I tell them I insert into the holes precut, prepackaged pieces of wood which are all the same size for continuity. They are impressed, and then leave me alone to do my work. If they ask to see one, I place a toothpick in their hands.
I have used very small, plastic hollow wall anchors for light loads on the boat.
They are quick and easy, but you probably will need to make the holes larger to have even the tiniest one fit, so, bill's idea is still better (zip, zip, zip, zip). The new holes, along with the old ones, will be covered by the mirror.
Ray