A suggestion: have you considered a repeated License-File field instead of License-Files? Almost all other “list” quantities like Classifier, Requires-Dist, etc use multiple-use fields rather than a single field with some delimiters (Keywords is not, but that’s a bit weird anyway, and I think setuptools is broken with respect to keywords and its delimiters anyway?).
Though, also, I do think it would be very unusual to have any sort of metadata that is a reference to a file in the package as opposed to metadata about the package. It seems like it might be a better idea to glom on to the sdist standardization discussion and suggest that all the license files go into a dedicated directory like .dist-info/license_files?