1. Clean PCB blank and spray with high temp black / dark paint.
2. Laser at low power / high speed to burn away paint around traces (isolation) or raster for removal of larger areas.
3. Etch.
4. Leave the paint / melt in place over the traces (as soldermask) and Laser again at higher power, to burn away paint / melt from solder pads. In fact... spray the etched board again with green paint (or whatever color) and laser that clear off the pads.
5. If you etched out a hole in the center of each through hole pad, and have a high enough power laser, you might be able to laser cut your holes through the FR4.
Now you have a nicely protected PCB. Don't tell me the paint can't manage the solder heat because there are very high temp paints.
This REALLY sounds to me like something worth trying.