That’s brilliant ! 
Can I add a small feature request - to allow the plot-negative file to be Gerber, as well as Excellon ?
The reason is to support the often-requested Negative Text feature
With this, users can assign a special layer for cut-out/negative text, and plot that layer separately, then append to the copper-fill-layer using the LPC append.
(with due care, of course, as this will cut into any and all copper data 
)
One example of Negative text conversations :