weeeelllll...
I'm not a programmer, but I'm part of the ScummVM discord. ResidualVM, the sister project of ScummVM is what powers the Longest Journey mod, and ScummVM is a modern day interpreter for old adventure games. You *can* do things like audio mods in ScummVM, and the dev team have done things like implement high framerate modes for Blade Runner and other things. The guy who rendered this out is part of the discord and that sparked a discussion about doing something like this in ScummVM. Generally speaking it is *possible*, but would require a lot of work, and possibly re-authoring a lot of things like walking meshes and such if the upscaled art changes things. Plus a re-write of how ScummVM handles loading images to support external images.
From what I understand, The Longest Journey mod just replaces textures, something that ResidualVM (built for 3D rendering tasks) can handle well, but live swapping 2D art assets? Especially for games that might be hard coded to have certain things working a certain way? Might be harder.
It's something they're thinking about, but don't hold your breath...