So it wasn't done through the bootcamp assistant (thats just a partitioner and a Driver downloader anyway)
I manually created the required partition through disk utility and installed and it seemed to like it.
10.8.3 Mac, 2011 iMac model with a stock 1TB Traditional platter drive.
The only thing special (and most likely not supported in any official way) is the manual partitioning.
Was just wondering if there was a way to trick Fusion in to creating the bootcamp VM so I can then mess around with the VMX.