The lack of conversion from Path to CString is really inconvenient, so I would also like to see a solution in std or in a well-established crate. However, I don't think defaulting to UTF-8 on Windows is a good solution. This is a problematic conversion, so it's better to force users to make an explicit decision about the approach.
Ideally, when passing a path to a library with C interface, you need to examine the library's documentation or source code to figure out which encoding it expects to receive. For example, some libraries explicitly declare that input must be UTF-8, and some libraries need the path to be converted to the current ANSI code page or OEM code page because they just call fopen with it. I think it makes sense to have a separate functions for these cases (and maybe there are other common cases). It seems that Windows API provides functions for this kind of conversion, so hopefully it can be done without dealing with all possible encodings directly.