A simple way (though there may be others) to do this would be to start out by creating 3 standard playlists and call them A-I, J-R, and S-Z. Then pull up ALL of your music and drag all of your artists from A-I to the first playlist, the J-R to the second, and the S-Z to the third. Once these playlists are created you will be able to create the desired genre playlists to be accessed through the RiverMote.
For example, you could have a smartlist matching all files in the A-I playlist with the genre of Jazz and so forth.
This will require you to update the 3 stock playlists as you add music, but all of the smartlists will automatically update once you have done so.