So here's how we did it - it was a super hack on the code side. Our sound guy produced a distorted version of the main theme. Not sure what he did but it sounded messed up and I can ask him to give you the specifics. However it was exactly the same length as the main track.
At game start I'm starting both tracks, one at zero volume. On activating slow motion I simply swap volumes on the two tracks. Combined with the zoom and the screen lightening I was so pleasantly surprised by how awesome it looked.