I want to propose an implementation of pg_import_system_collations() for WIN32 using EnumSystemLocalesEx() [1], which is available from Windows Server 2008 onwards.
The patch includes a test emulating that of collate.linux.utf8, but for Windows-1252. The main difference is that it doesn't have the tests for Turkish dotted and undotted 'i', since that locale is WIN1254.