You'll need to install lyricwiki and appscript before you can use this script.

itunes_lyrics_fetch.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from time import sleep
import lyricwiki
import appscript

it = appscript.app('iTunes')

def fetch_and_add_lyrics():
    track = it.current_track()
    if not track.lyrics():
        lyrics = lyricwiki.get_lyrics(track.artist(), track.name())
        if lyrics:
            track.lyrics.set(lyrics)
            print "UPDATED lyrics for ", track.name(), 'by', track.artist()
        else:
            track.lyrics.set("None")
            print "Unable to get lyrics for ", track.name(), 'by', track.artist()
    else:
        print "Lyrics exist for ", track.name(), 'by', track.artist()

    # Uncomment it.next_track() if you want to quickly go through current playlist
    # and attempt to add lyrics for everything…
    #it.next_track()

try:
    while True:
        if it.isrunning():
            fetch_and_add_lyrics()
        sleep(1)

except KeyboardInterrupt:
   print "Bye!"