On the PC, MP3 Tag Studio is generally the most powerful tool for applying changes to groups of tracks all at once. I don't know how the files are organized on your hard disk, though, so it's probably going to involve a certain amount of fiddling around if you go that route.

On the player, you could do a search in emplode for where Genre = "" (not sure the syntax), and then sort by the artist column and multi-select groups of tracks and edit them that way.

Oh, by the way, when I tried it there was still a bug in Jemplode that didn't write the Genre or the track number properly to the tags, so you might want to more seriously consider the PC-to-Player option.