[int_19h]

In principle, why wouldn't it work for C++/WinRT, given that it doesn't use the WinMD metadata directly, but rather produces valid C++ headers from it?

I think the problem for Zig is that they use MinGW as their Windows target, and the latter's standard library isn't really UWP compatible (yet?).

[pjmlp]

Because the whole build process to produce a MSIX or APPX requires a little more, like the whole Windows SDK, plus whatever might be fetch via NuGET via MSBuild, and the process to sign the packages?

[int_19h]

I would compare building MSIX or APPX to building an apt or rpm package on Linux - that's the next step after building the actual executables, which is what Zig covers.