If you didn't care about having a fully-redistributable image, but were okay with doing things more like modern old-console-game decompilation projects do (where compiling the project requires the original game ROM as an input), would a project like this be more trivial?

I.e. how hard would it be, comparatively, to design a virtualization framework that doesn't actually ship with any ground-up-rewritten libraries, but instead just expects to execute the binary in question in the context of a full rootfs copied over from a "real" macOS install?

with games, afaik that is mostly necessary for assets. For an OS, this would basically like a VM and is slower/more overhead.