distutils will install the headers to {base}/include/python{py_version_short}{abiflags}/{dist_name}, and I think probably the best option would be to do the same. So, I think we could add a site-include scheme like I described and have the installers append a directory with the distribution name to it.

I think it would be reasonable to put all headers in a shared directory, site-include, given that we make sure the installers put them in a subdirectory for each distribution.

About standardizing pkg-config-like metadata, I don't think that's a great idea, as it would not replace the mechanisms you mentioned. It might replace some, but not all, as there could always be a use-case that needs more information or interaction than what our standard may provide.

Does this make sense to you?