Are separate display devices associated with headless endpoints a thing, and if not, can they be?

Yes, sure !

But … not so easy when using headless endpoint (MicroRendu for example). I know it is possible to use on the same zone an Android (or IOS) tablet on a support (grouped with main audio endpoint). However i would prefer a cheaper solution and form factor like Squeeze Box Touch. Perhaps emerging Roon API would permits such devices to be available.