What if you have a bluetooth connected vertical bar that upon activation with the phone sweeps the length of the board and then goes back. For this you would need a mechanical implementation such that it's attached to the board, maybe wheels on top and bottom edge of the board. There are chalkboard erasers you could put in a line, maybe you could make them replacable. Maybe on a whiteboard sometimes a cleaning spray is necessary, this could be automated, either sprayed onto the surface of board or the sweeping arm itself.