I was thinking about this and have a theory...
In the Auto industry there's a liquid mask by 3M called Overspray Masking Liquid. Essentially its a water based liquid mask that dries to a thin film that will protect just about any surface from paint oversray and its water soluble. My theory is that you could coat the surface with this stuff then engrave, spray paint with a can of spray then when the paint is dry wash of the remaining mask... Job Done!
Besides 3m there are lots of products out there that are used as liquid masks including the building industry that work on the same principal but I think finding one that's water soluable is the key. I also had the thought of mixing my own with some flour and metho with a bit of food colouring to make a brushable quick drying paste to act as a mask. Think here of the Cermark spray for Stainless Steel. When dry its a brittle coating that just washes off, yes totally different but I'm thinking here how it dries and sits on the surface.
Anyway all of this is just theory and I haven't tried anything, but I thought I'd chuck it out there because sometimes the simplest things can lead to solutions.