If its any help this is what I have done in bCNC for laser work:
I have added 4 user defined buttons on the control tab:-
Laser mode on; $32=1
Laser mode off; $32=0
Laser constant power mode; M3 (Laser only switches on during G1, G2, G3 off during G0)
Laser dynamic power mode; M4 (Laser power varies with feed rate automatically)
I set the Z to zero when generating the G-Code. Then before I start the job, I just press the laser mode on button followed by the laser mode I want to use. The software takes it from there. Don’t forget to select laser mode off when you’re finished as it is stored in the Arduino and will still be active the next time you use it.