Thanks for replying.
In answer to your question:
Far-seeker:
Why not make a stencil the exact size and shape you want?
The robot will be printing binary images sent to it on the fly.
Far-seeker:
You might even be able to get away with using paint from a spray can, if the robot is able to press and depress the top of the can quickly enough (something that a solenoid with a return spring could potentially do).
This could potentially work, except that we need to use a specific type of ink that is easily erasable. Maybe there's a way to get that kind of ink/paint pre-packaged as an aerosol.
Thanks again!