You can potentially use the pen-up/pen-down events in the EggBot control scripts to turn a laser on and off through the EBB.

What I’d suggest is changing the definition of the pen-up and the pen-down commands, to turn the laser off and on, respectively.

The EBB command set documentation is here: http://evil-mad.github.io/EggBot/ebb.html
One of the “fine print” things there is that there a number of “UBW” commands supported, which are documented here:
http://www.schmalzhaus.com/UBW/Doc/FirmwareDDocumentation_v145.html

You’ll want to use the “PO” command from that set, for example “PO,B,0,0” to take pin B0 low, or “PO,B,0,1” to take pin B0 high (I have not tested these commands while writing this — just reading from the docs). Pins B0 and B2 would be particularly good choices, since they are not otherwise used on the EggBot/WaterColorBot/AxiDraw software.

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.