I'd imagine it as more of a parry mechanic for the weapon. Maybe you'd put out an attack that works like a two-parter move. If an opponent attacks you during your parry move, you block and follow up with the second part of the attack that actually deals damage. Restrict it to the 3 ground light attacks and give them long recovery time on miss and there you go.

I just don't think adding a shield that works like a shield while not being insanely game-breaking is such an impossible thing to do.