Since was playing with the brickpi earlier, tried an eggbot:

as with most things, the software part is easy for me, the actual pen/egg bits are more challenging.

Found the “gear play” and some friction issues, the egg slipping in the holder, or the pen pressed too hard and pushed the egg out of the correct alignment.

There is some feedback possible to detect the rotation of the NXT motors, but my initial version does not do this.

Did make some easily changeable pen holders:

so once I can get the software/penup calibrated, should be able to make multicolour patterns.