Xpantastic!

I agree there will be delays, but I'm able to download firmware via
USB from Windows 7 in Parallels running on OSX 10.6,
to microcontroller development boards that are external to the machine.

Granted it's not a midi device, but a straight USB interface is likely
more demanding than Midi.

Also, Xpander patches are sent/received via sysex, which is extremely
slow to begin with.

Give it a try- I'd be interested in your results. Just make sure that
you assign the USB device to the VM, so that it
doesn't get assigned to OSX..

I may just try this myself.