Could chainmail underwear protect you from a Taser?
The obvious answer here is, depends on where the taser prongs hit you. But, as stated, the question answer is probably: no. The statement of "if it were earthed" is not conducive of a straight answer, because you might be earthed too, and current will always take the path of least resistance, which might be you, or the chainmail - however, the chainmail is a conductor against your skin.
It might be better to merely add a protective layer that is an insulator between the taser and yourself, that the taser hooks cannot penetrate, thereby eliminating the taser from the equation.
The even more obvious solution is to find a way to resend the current back down the taser connectors to the taser gun, either shorting it, or causing a shock to the person firing a taser at you. I don't have an easy remedy to that one, however.