Anyone know why they didn't provide for multiple generes in the MP3 tag format?
It is provided for in the ID3v2 tag format if I recall correctly.
Is there a way around this for use in the empeg?
Yes, create free-form playlists containing whichever tracks you want. You can name the playlists anything you want, including the names of genres, and copy a given artist into multiple playlists.
Also, are the soup views going to make it into a future version of car player software?
That's the plan, although not in any immediately-pending releases, it will be a post-2.0 feature. And in any case, it wouldn't help your multiple-genre thing.