I'm a helmet-wearing libertarian and skeptic.
You presume not wearing a helmet increases the rider's health care costs to society. I'll bet it doesn't. First, helmet-less motorcyclists involved in serious crashes are very likely to die, which is likely cheaper---in terms of health care at least---than treating injuries to helmet-wearing riders. Of course, in some cases, the helmet-less rider would suffer more serious injuries than helmet-wearing rider, so it's not clear which effect would dominate. Second, any helmet-less riders who die while they're riding avoid the high health care costs related to old age and dieing of natural causes.
Is it clear whether not wearing a helmet increases or decreases a person's lifetime expected health care costs? No, but I think it's likely that it decreases the costs.