Hi Darius,
You're quick. I was just about to reply here that I got the same problem under Fusion 5.0.3 and was about to go test in the WS TP, so you're saving me that step.
Problems with the USB stack? Oh.. ironic as in this article they say that it is one of the areas in which they made significant progress. 
Retested with USB controller removed and now the VM does indeed run under the Fusion Tech Preview.
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Wil