Some fonts are available under Windows without being in the C:\\Windows\Fonts directory. They are listed in the registry key
See http://tug.org/pipermail/luatex/2015-May/005212.html for a few links.
I think fontconfig should be able to find them, as they are available in other Windows applications.
See https://github.com/phst/lltxplatform/blob/master/src/fonts_windows.c for an example of code listing these fonts in C.
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