As for a physically present attacker, many projector systems are out of reach and have a cable running through the wall or table to produce a socket for users t

As for a physically present attacker, many projector systems are out of reach and have a cable running through the wall or table to produce a socket for users to plug in to. Or they are mounted inside a cabinet with a similar cable reaching to the outside. If the "firewall" device is present at the (inaccessible) projector, this would not be a problem.