If you change state_type in the char_traits, you'd be able to differentiate the various basic_string types and include information about the character encoding without writing a whole lot of new code.
Thanks for the suggestion. I need to learn some more about this corner of "namespace std", clearly, before I go and re-invent something. IIRC, some of the non-const std::basic_string methods aren't suitable for handling variable width encodings like utf8 and utf16 - non-const operator[] in paticular returns a reference to the character type - a big problem if you want to assign a value > 0x7F (i.e. a character that uses 2 or more bytes).
I've noticed that there are frequent requests/proposals for some sort of boost unicode/string encoding library. I've thought about the problem and it seems to big for one person to handle in their spare time - perhaps a group of us should get together to discuss working on one? I'd be happy to participate.