You would be surprised how far the laser will focus. Try it before you worry about the gcode complications, gives another reason to play with your machine. just set the perfect focus at half your cut height and then you will get a nice +- range.
I think I will be doing this for my 5 year old shop assistant. I want to mill him a wooden sword and etch in all kinds of designs and his name. I will make a video if I do.