I have witnessed many times the fielder's mask preventing severe facial injuries.I have also witnessed players receiving severe facial injuries that a fielder's mask would have prevented. I have yet to witness anyone being hurt directly by a fielders mask, ever. (Someone chime in if you know of anyone that has been hurt by a mask)
I am very comfortable standing in front of a judge and jury when some lawyer is trying to take me to the woodshed on requiring a mask. Liability is a red herring and just an excuse not to anything. It is always brought up as a way to derail a discussion on doing anything new.
What would a mandatory mask rule add to the cost of ASA's Insurance? Wouldn't even ripple the surface.
Sure go ahead and get a safety ceritfactation like on a facemark on a batting helmet, but don't hold up the requirement on account of liability. I'd say they are just as liable by not requiring it.