In the time of pandemic, our parishes are getting instruction on distributing ashes two weeks from this writing.
Sprinkling ashes on the heads of penitents was the first suggestion I saw a few months ago. A later complaint: ashes in eyes. Less so in hair, I think.
In my diocese, a suggestion was to use cotton swabs on the end of wooden or plastic sticks. Not to be reused, I gather.
There is no requirement to go to Mass on Ash Wednesday, nor to receive ashes however the bishop or pastor determine. That said, which form of imposition would you adopt, were you to sit in the liturgist’s purple chair? Or would you suspend the penitential ritual this year? Do tell.
(Atop the crown of the head is, historically, probably older and formerly more common than the forehead.)
Sprinkling the Ashes is my vote.
Though in all honesty, it doesn’t matter whether it is done via sprinkling or via marking on the forehead. The main route of transmission is that you have to be within 6 feet of the minister. If all are masked and the interaction lasts less than a minute, than there should be minimal risk no matter how you do it.