I like this, but it should mention that this is not always possible.
Some header-only libraries install stuff to %{_libdir} (e.g. cmake files
or pkgconfig).
You’re right. Considering this, “many” header-only library packages
could benefit, rather than “the vast majority.”
Of the twelve header-only libraries for which I am primary maintainer
(atomic-queue, debugbreak, dr_libs, earcut-hpp, fast_float, geompp,
gulrak-filesystem, hedley, lexertl14, pcg-cpp, stb, wyhash), nine look
like they could benefit from this technique, and the remaining three
could not because they install CMake files to %{_libdir}/cmake.