I just had an idea: It would probably be feasible to implement a way to install and run any Flatpak-packaged desktop Linux app on Windows and macOS inside a micro-VM, using a virtual machine host like Firecracker, or more likely, Firecracker's progenitor crosvm, which was designed for running an arbitrary Linux userland on Chrome OS. I believe crosvm can even run on Windows, using the Windows Hypervisor Platform (also called WHPX).