Further to the topic Enable drill marks for Gerber files I created a script to combine gerber files with positive or negative polarity. You can find it here https://github.com/bobc/kicad-utils/tree/v5/scripts/gerber_combine. Note that the gerbers files must have the same format (ie… inch/metric, number format) as no conversion is performed.
For the use case of negative text in a copper fill, you might do something like:
python gerber_combine.py ktest4-F.Cu.gbr -s ktest4-Cmts.User.gbr -o ktest4-F.Cu.merged.gbr