I'm not questioning that. Only an asshat would try to file workman's comp for an injury that happened while skiing in a sanctioned race not being paid on the clock at the time. What I am questioning is could they deny paying the healthcare benefits/medical costs of injuries from an accident while free skiing? Could there be a medical insurance coverage denial for high risk activities? Can the employer say the employee voids their medical coverage if they choose to ski back country and get injured?
The answer to that is not really. First off, the vast majority of ski instructors do not receive health insurance from their mountain. As seasonal employees, they are not required to be covered. So most are covered either by their "real" job, ACA exchange/Medicaid, or by a family member's policy.
Even for those who would receive benefits through their mountain, the employer does not have access to your medical records. The insurance company does, as they are the ones paying the claim. However, due to HIPAA regulations, the insurance company cannot disseminate any personal health information (PHI) without consent, unless there is a clinical or medical billing reason to do so. So the only way for the insurance company to deny a claim would be if the activity were already specified as an exception to coverage. Since the ACA has come into effect, the ability of insurance companies to include and enforce exceptions has been significantly limited. And the writing of policies and exceptions is something that is done by the insurance company, not the employer.
So long story short... could an insurance company conceivably deny a claim due to an activity such as backcountry skiing? Yes. Is it likely? No. And its not something that can be done after the fact. It would have to be in the policy upon inception as an exception to coverage.