Hey, I got a definitive answer from the relevant Fusion developers.

This is a bug in Fusion. It is not fixed in Fusion 1.1. FYI the reason I thought it would work was because the same trick (enabling the undocumented/"experimental" USB mouse support) does allow you to use more buttons in our Windows and Linux products (Workstation/Player for Windows/Linux) which share most of the same code. I know that's not of much interest or consolation to you.

Anyway, we're now tracking this bug internally. It should be relatively isolated and we'll try our best to get this fixed in the next Fusion release after 1.1 goes out the door.

If you're feeling really adventurous and/or desperate, you can take out the mouse.vusb.enable line and add this option instead:

usb.generic.allowHID = "TRUE"

Then, you'll notice that your main mouse and keyboard (if they are USB) are available to pass through into the guest via the USB devices menu.

The dangerous part here is that once you pass through the mouse, it is actually disconnected from the host, so you won't be able to ungrab from the guest just by mousing out of the Fusion window. You can still ungrab with the keyboard (ctrl-cmd I believe is the shortcut?). If you actually pass through your keyboard and your mouse, you'll be stuck in the guest and you'll have to shut it down (or worse, reboot your physical machine).