I had a look at Grbl code, if you disable VARIABLE_SPINDLE then D12 becomes spindle enable. I don't know if this would work correctly as T2Laser may not be able to set spindle 0 for off and keep the enable pin on, so you may find it's no help.
Manual is problably easier but I am sure there's a way to do this, perhaps using the stepper enable D8 (this is nice as you can set a time out after all motion stops). The other option, and I suppose the correct one would be to use A3 (coolant) but unless you add M8 and M9 to the G-code it wouldn't be automatic. You could easily setup custom buttons for it or in a future update I can add an option to automatically enable and disable.