With regards to the shout I made yesterday, you're right, you need an Elite's serial number to request a new cable. I had a look on the xbox forums, though, and
there's a few people saying you can call the microsoft customer services and they'll have one sent to you if you explain the situation. You might need to call a couple of times until you get a sympathetic operator, but if nobody here has one then that's the way to go I think.