My expertise on Windows is rather limited, but as far as I understand the issue, I consider this a reasonable idea.
I think it is impossible to find a perfect default encoding, and utf-8 seems to be the best bet with regard to portability. IIRC most of the archivers on the Windows machines I have access to use latin-1, but I don't think that latin-1 is a suitable default value. I don't know much about Windows internals and have no idea what mbcs really is, but it is actually not available on other platforms.