Frew, you’re talking about using the laser to expose a treated PCB, which is then etched. Basically the same process as used to make a plate for a printing press, except Copper instead of Aluminum or Zinc or Steel.
The OP looks like he wants to use resin printed on a blank PCB as his etch-resist. He’d then etch the board and remove the cured resin to expose his circuit… so he’d need to mount his PCB to the bottom of the build plate (or a substitute build plate) and print directly to the board. I suppose he could just stick the board to the bottom of the build platform and use the fine tuning to move Z up by the thickness of the board. But I don’t think it’ll work. I bet resin won’t stick to copper very well.
Your idea is better if for no other reason than it’s probably a lot less messy.