Can I be charged a co pay for a doctor visit on a workmen's comp claim?

No.

If you told the care facility when you arrived that you were there on workers' comp (which is normally a question they ask up front), you cannot be given a bill for copay or anything else because it is the insurance policy of the employer, and not your personal policy, that pays the bill. Will not affect your policy at all, because the employers' policy is the responsible party.

If, however, you didn't tell the hospital or dr office and they billed your personal insurance policy, good luck. By signing that bill authorization, you assumed the bill and liabilities of it and the employer can be Scott free unless you have proof that it should be workers comp.