Fair enough; but you could use diazo sensitizing which is not toxic, and if you're vegan you can use PVAc (polyvinyl acetate). But.. I digress.

In the photo world, polyester film = PET/Estar/Melinex/Mylar

So what can we use to etch this?

I can't think of an easier way to make a relief image than by, generally speaking, cross-linking a colloid. Along your line of thinking, why don't we just use a high-powered laser to melt the polyester? That seems a lot easier than messing with chemicals and inkjet nozzles. Besides, any chemical that's going to destroy polyester is going to be a very nasty solvent, with terrible fumes.

Lastly, densitometry won't be difficult since this will be a high contrast screening process.