What if the pen changer was three plates. Two upper plates (pen holder) to hold the pens. It would be in the shape of a spool so it could sit on the selector plate and have a series of holes for the pens. The selector plate at the bottom would only have one hole so only one pen can go down to the paper.
The idea is to lift the pen holder, then rotate it so the right color pen is above the selector hole. Then it can be lowered so that the selected pen will go through the selector hole. The rest of the pens will be prevented from moving forward.
Pens would be “spring mounted”. I’ve shown a pink rubber band to represent this. A pen would be mounted in the pen holder so that it tries to be in the writing position. It can give either way since there is some sort of spring or rubber band holding it.
So when the pens are lowered, the selected pen goes through the hole and the rest are blocked.
1) A pair of servos could probably do this. One to rotate and one to raise and lower.
2) There could be a few pens and this system could select and also raise all pens to move the gondola without making a mess of the drawing.
1) I don’t know how to make the pens idle up and down smoothly in the pen holder.
2) Dangling the servo wires down from the gondola may not be so elegant and it may have some impact on the freeness of the gondola. My gondola works better with a ballast so this may not be too bad.
Perhaps each pen holder could be a brass tube that can idle up and down easily. The tube could have a couple springs to make it have a preferred position and able to be pushed up or down. Then the pens could be placed inside the brass tubes to mount and unmount them. Perhaps some putty or a clamping system (set screw?) could be used to mount them.