Thanks for your reply, Thorben.
You're correct, Windows Virtual PC also has a setting for choosing which USB devices are routed to the virtual machine. However, this doesn't seem to include mice (these are connected to both the host and client automatically, but only as standard PS2 mouse).
In below screenshot of the XP virtual machine it can be seen that there are only two other devices (a scanner and a USB sound card) that could be routed to the virtual machine, but no Fingerworks device). Thus, the Multi Touch Utilities can't find the iGesture Pad (which however is connected and working in the virtual machine as a standard mouse).
Amazingly, after installing the Multi Touch Utilities in the virtual machine, they even can be launched directly in the host system (Windows 7) like shown in below screenshot:
(here, Multi Touch Utilities runs two times, one via the virtual XP machine (see the XP titlebar design!), and another instance running directly in the Windows 7 host). Both however don't recognize the iGesture Pad
Anyway, for the time being it looks like I will keep a real Windows XP installed on a hard drive somewhere, just in case.