Theoretically it's possible.
MP3's store text information, aka. ID3 tags. Here's where you find information such as Artist, Song Name, Album, etc, hell, you can even store virus code in there. Here's where lyrics are stored, as text.
Now, Spotlight does index at least Artist and Song Name, so indexing lyrics is obviously possible, what I don't know is if Spotlight actually cares for any other tag than artist and name when it encounters a media file.