Typically there would be two different approaches to writing lyrics. It's either lyrics first - possibly with some kind of tune in the mind that will not necessarily play any part in the further process, or music first. Actually, the first approach means that you write poems that are set to music some time later, except that the poems are meant for music from the beginning, i.e. they would have a characteristical structure that predestines them for music like repetitive parts for the choruses. Most lyricists prefer writing their texts without any restrictions. It's much more difficult to find matching words for an existing tune.
Now that I've begun working for Karl-Juergen im confronted with a third approach: writing lyrics for chord progressions. There is no vocal track, i.e. a tune for the vocals, there are only the harmonies. In a way they sure define the tunes that may be viable, they set restrictions as to the key, the length, the rhythm, or even the mood. I think, beginning with a "cool" riff would make a similar approach, the music sort of defining itself. AC/DC probably create their songs that way (maybe you could argue that they don't have melodies at all, just cool riffs and chord progressions).
How I'm going about writing for K-J I will post here shortly.
I started a similar project on my own a while ago. It began as a little exercise for playing power chords and its result was the song "holy hooker" which has not got too bad at all, considering... (besides the power chords it comprises a rather nice riff). Here is its history in my blog:
power (1) power (2) a subject for "power" (3) notes for "power" (4) holy hooker The recorded song is on the playlist of my
MySpace site (currently on position 7, but that may change as I'm adding songs). If you like it (unlikely, but such things happen) you might also fancy "long-legged divinity" ;-)