The imposition of ashes is a human ceremony, although an ancient and respected one that finds Biblical precedent. It becomes what we make of it. Use in private devotions should be fine, if so desired.
Some thoughts on ashes for Ash Wednesday.
The irony of being concerned about mortality and avoiding possibly spreading a mortal disease while participating in a ceremony to remind of our mortality is not lost on me. Yet the life that we are given here is itself a gift from God even though inevitably limited. The reminder of our limitations and reason for those limitations (sin) is a salutary one. But as God's gift to us, our life here is not to be squandered but treasured and used wisely, not arbitrarily cut short nor recklessly endangered. To be reminded of our mortality (dust you are and to dust you shall return) while seeking to assure that the reminder does not itself threaten to hasten that return to dust may be in a way ironic, but also reasonable precautions shows respect for that gift until God brings it to an end.
The imposition of ashes is a purely symbolic act, with no divinely mandated directives for the action. Christian freedom suggests that we may modify as we see fit for our immediate circumstances so long as those modifications do not turn it into mockery of the Gospel of which we should be reminded.
As to going about with the visible sign of ashes, does that violate Jesus' prohibition of making a show of our piety? What was Jesus' intent? Seems to me that Jesus was against those who would use visible signs of piety as self-promotion, "Look at me! look at how pious I am!" Wearing ashes, or a cross for that matter, to draw attention to my own faith and piety is condemned by Jesus. However, I doubt that many do so. Rather it is a personal reminder, and a witness to others not of MY faith, but of Jesus and His solution to my sin. Know yourself and why you show visible signs of piety. Self-examination is as difficult as it is important. We can often fool ourselves as easily as we try to fool others.