Stencil for PCB etching?
No, it wouldn't be possible to make a one-piece stencil except in the most trivial designs, in which no copper completely surrounds any other copper. You certainly couldn't do a board that has a "ground pour" surrounding all of the circuitry &mdash; the center of the stencil would simply fall out.

Besides which, if you have the technology to make the stencil, why can't that technology be applied directly to the board in the first place?