why not just add three microphones to your table and use a dump stick as your pencil. the sound waves are delayed proportional to the distance. by correlating the three signals you are able to get the distance between pencil and microphone. knowing the position of your three microphones you are able to calculate the position of the pencil.

i think there was a demo with piezo microphones on a wall or chipboard, but i'm not able to find it.