Awesome! Glad it worked for you!
Any chance the layer could be simply edited from F.Cu to “Keepout area” in the file? Or is this impossible?
Is it possible to do the ‘inverted copper’ as shown?
Not directly, but the expanded Gerber standard supports black and white (positive/negative) plotting, so you could plot the text on a separate layers and then instruct the board fab to add as negative to the coppers, or you could manually edit/append the copper Gerber files with the negative header + text.
Doing this, the user (of course) needs to take care to ensure the negative text does not cross any traces
Not sure how to do this. Does anyone know what layers/objects in KiCAD would create this?