Without prejudice to the question of running *non-approved applications* within XPMode, the special integration of XPMode and Windows 7 in the Windows Virtual PC does expose some of the XPMode installed applications to the Host Windows 7 machine - Microsoft Office, Microsoft Security Essentials, Powertoys for Windows XP - anything from the Microsoft stable seems to pass through appear in the Windows 7 All programs, under Windows Virtual PC, XP Mode Applications.
I even tried an unlikely installation of Microsoft Virtual PC 2007, which installs fine on XPMode, but will not run as a VM inside a VM -I did it to obtain for the virtual disk precompactor, not available with the Windows 7 VirtualPC, and, yes, it shows in Windows 7 All programs.
However, so far, no non-Microsoft program (Google Chrome - nothing else installed) is detected. No programs in other Windows VMs -Windows 7, Win2K show through at all.
Non-installed programs running from a USB stick, for instance would be undetectable in many cases.