Songwriting angst.

I'm currently trying to write a soaring piano intro to a rock ballad, with gorgeous chromatic scales leaing to a crescendo where the guitar will join in with a squeal of feedback.

Main problem is, the tine I've got in my head is currently unplayable by anyone with the standard number of arms. Does anybody here with classical piano experience (I haven't played any concert piano stuff for about ten years) have any advice for playing very fast descending runs as octaves? I find I have lots of trouble keeping my little finger and thumb exactly one octave apart when moving between the keys.

Hot piss, I bet Matt Bellamy doesn't have this problem. Talented little shit.