Without reading further on ... this is the right way.
Mp3tag uses the "\\" and foobar uses the "; " as a replacement just for displaying the binary separator for multiple values which otherwise cannot be shown to the user.
In the moment when a user writes a string "\\" into a Mp3tag field this string means nothing more than the string "\\" itself. In the moment when the complete tag is written to the file Mp3tag uses ID3 standard rules and converts this placeholder into the correct binary representation.
Other ID3 conform applications can read such binary values and use their own replacement character or string for displaying the mutiple values at the screen.
DD.20091128.1148.CET