You could park it on a busy and preferably narrow road with the mirror in unfolded position and wait for a truck to come by at full speed "gently" encountering your mirror. When the mirror reaches the farthest end towards the bonnet of the car, mother nature will apply all physics available so that the cover will conveniently pop up for you ready for extraction.
Sorry for the humor, it's just that this question reminded me of such an encounter.. Good thing the cover did not break, I was able to just click it back in position.