The '\0' character is not valid anywhere in XML, in any encoding. I don't know the reasoning but it means you have to use some kind of alternative representation for data that could contain NULs.
If you're talking text strings with embedded NULs then you might need to define an entity that can stand in for the NUL, so you can expand it back to NUL when you recreate the string from the XML archive, or put all strings that might contain NULs in an element like <hex> and hex-encode the bytes. There might be other solutions too, but I've not used them.
jon
Basically we want to map anything that might be contain in a std::string or std::wstring to an XML value string. I little investigation makes me think that the appropriate mechanism is to escape all "non-printable" (uh-oh?) or some subset of "problem characters" using the % escape syntax. It looks to me as non-obvious problem but I have yet to delve into it. Robert Ramey