The way to do it, use clear acrylic, paint it white all over, even the back (make tiny holes in the back for light diodes) now the light will reflect off the inside of the white paint (better light distribution),
Now paint the front black (even the back if you like), now where back at your big problem!
The problem with acrylic, is that itís not the same thickness all over, the idea is to remove just the black leaving the white paint.
The guys over at Viperpits Viperpit.org Second Edition ! have dropped CNC engraving and resorted to Lazar engraving, what you need is a way to scan (probe) the surface, just like PCB engraving.
Iím sorry I canít help more, actually I would like to do the very same thing myself.
I will be following this topic.