You need an extended version of that board that allows a flash ROM to be written over an on-board serial port from another machine without pulling it out of the Mac. (Or, hey, even better: hijack the power-on hardware initialization to jump to a "recovery loader" which checks to see if some kind of switch is held down on powerup and, if so, initializes the Mac's SCC and accepts a new flash image over the built-in serial.) Then imagine all the fun you can have writing bare-metal software for the Plus!

(Hack together some solution for interfacing a joystick and port video games to it. It already bears a striking resemblance to a Vectrex....)