Why not make a stencil the exact size and shape you want?
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).
http://nicholasclewis.com/store/http://spritesmods.com/?art=inker&page=2http://reprap.org/wiki/Reprappable-inkjetProbably find lots more examples with a little google.
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