Is it technically possible (I don’t think this would be restricted), and how do you go about it?
I got my little test app building fine, but when I run it I get a failure dialog.

The debug output is slightly more informative:
The thread 0xe4c has exited with code -1073741515 (0xc0000135).
The program '[3400] OpenCVTest2.exe' has exited with code -1073741515 (0xc0000135) 'A dependent dll was not found'.
Then I created a Win8 Desktop app, which also built fine but failed to run.

MSVP100D.dll is part of the VC++ 2010 Redistributable which is not compatible with Win8. (Being a hacker I tried it anyway, the installer runs to completion – but the bits aren’t installed.)
Is there a compiler flag I can try? Possibly a custom build of OpenCV would fix it, but that would be a last resort, and it might only fix the desktop app.
This guy shows an example of using OpenCV library for performing face detection in a Windows 8 Store app written using C++/CX:
http://channel9.msdn.com/Events/TechEd/NorthAmerica/2012/DEV322
Also, at the below post he also describes how the C++ Runtime works for Windows 8Store apps:
http://blogs.msdn.com/b/vcblog/archive/2012/09/28/10354327.aspx