>Copper itself has antimicrobial properties (studies say SARS-CoV-2 only lives 4 hours on copper), but does anyone know if there is enough copper content in those fabrics to make them a better fabric for face masks than cotton?

I think you answered your own question. Viruses only get killed if they're on copper for 4 hours, so it might be useful if you wanted a self-disinfecting mask, but if the material itself doesn't filter properly the antimicrobial properties of copper doesn't matter because the viruses fly straight through and aren't affected by the copper.