I have never looked in to this, however I am not sure if this will work, and I think the only way of finding out is to try it.
The fact is, in most VM software I have seen that support USB pass through, it is achieved via disconnecting the device from the main machine and then putting it through to the guest.
Items like keyboard and mice do not apply here as they are either not done through USB or are not supported via the VM software. That being said, every other USB device is (on my laptop, I can pass through the bluetooth, webcam and a couple of others).
It depends on how your multi touch is configured. If it is a usb connection to the PC, then by passing it straight through to the VM*, you should get multi touch inside it (but will need calibrating amongst other things (and I imagine will only work full screen), however if it is connected as a standard mouse, you may have problems as every VM software I have seen only supports standard actions via it's console and is translated to mouse actions.
*just be careful, if you have no trackpad and the multi touch is the only mouse, you may loose it and will have to come out via keyboard shortcuts / Ctrl+Alt+Del
So, try it! you have nothing to loose.
I would also say, I know that the Microsoft Surface SDK has a touch/surface emulator that lets you try multi touch based commands that you may want to take a look at. I have not looked at the Windows 7 SDK but as Microsoft are trying to promote multi touch, I would assume it is included there as well.