One of the most famous, most covered songs in history was created this way. The composer woke up one morning with the melody in his head, and sang "Scrambled eggs, oh you've got such lovely legs" as the lyrics as he developed the song. He eventually figured it out, and that's how Paul McCartney wrote the song "Yesterday."
For a good example of this technique, listen to Peter Gabriel's "A Different Drum" starting at about 1:40. His "lyrics" are just word-sounds.