I know this is an old post, but this may help someone in the future so…

I like to attach lasers to drawing robots as well :-)  so the way I have done it on more than one such machine (and the same way I put a DC motor engraver on the eggbot before there was official support as well) was by simply placing a microswitch in series between the laser driver and power and installing the microswitch in such a way that when the servo brings the pen arm down, it closes the switch and pen-up opens it. It’s low-tech and less elegant, but it was a very quick way to accomplish it.