Its true that brine makes the skin tougher. The technique of soaking hands in it goes back to the bare knuckle days. You develop calose hands and tough skin on your hands and it will be there for life. Which brings up a point...do you really want to do this to your face. Think about it guys...you won't be boxing all your life. You make your facial skin like leather, and exposure the sun, lack of moisture, scuffs and cuts, and a multitude of other things, will have you looking like your 70 when your 30.
Training with head gear is there for a reason...no amount of 'toughing' facial skin will substitute for that protection. If you go pro without the gear, it's not going to offer any more additional protection.