Last time we did a test to see if solder resist film can be laser engraved, see https://hackaday.io/project/191904-laser4diy/log/221159-solder-mask-laser-etching

That looked quite promising, so we went one step further and tried to put a solder resist mask on a PCB we created with Laser4DIY:

That worked...  but not exactly great. :-(

First of all, this PCB is tiny. It's our Laser4DIY Coin made by Alex, measuring only 17mm in diameter. So the pads are tiny as well and that caused issues:

• The registration is off. We tried to do it using the red pilot laser on the CO2 laser cutter and that is not precise enough. We have to think of better ways
• The pads are too big in the solder mask. I guess this is caused by the laser spot size. The resolution of our CO2 laser (we used an Epilog Zing) should be sufficient, but simple engraving the exported solder mask pattern does not do the trick. We have to shrink the engraving pattern to compensate for this.
• And finally, we got ugly dark spots during the process. At many places where the CO2 laser was engraving on previously ablated copper, the underlying FR4 was burned and at some places even causing defects on the copper. We hope that this is a non-issue as soon as we got registration and engraving area right, because if the PCB design is correct, the solder mask should only be removed on copper areas. 

Definitely need more experimentation in that area...