Rubber gloves? Q-Tips? Imposition with immediate removal with an antibacterial wipe? All sorts of things are surely possible, but perhaps we should look first to see if the tradition has anything to teach us.
The implication of the questions has generally assumed that some sort of fresh and innovative solution could be found. Several bishops have recently asked if I had any counsel with respect to the imposition of ashes in a time of pandemic. A Note on Ash Wednesday and the Imposition of Ashes in a Time of Pandemic