The ID3 tags only allow you to assign 1 genre to the music

That's not true. From the ID3v2.4 Frame Definitions (which is currently not available for me, so try the Google cache):

 TCON

   The 'Content type', which ID3v1 was stored as a one byte numeric

   value only, is now a string. You may use one or several of the ID3v1

   types as numerical strings, or, since the category list would be

   impossible to maintain with accurate and up to date categories,

   define your own. Example: "21" $00 "Eurodisco" $00



   You may also use any of the following keywords:

   

     RX  Remix

     CR  Cover

Of course, the empeg and emplode won't understand that, as they don't have support for multiple genres, but it's readily available in the ID3v2 spec.