This is odd - I tried it on my VeSys 2.0 installation and the "Harness" attribute "visible" checkbox is available and I can check/uncheck as needed. Can I suggest that you raise an SR, customer services might spot the difference (we can then update this thread).
I have VeSys 2.0 2010.2 SP1107 installed, and I can see the Harness value ungreyed in the General tab. I can then use the dropdown to change the value and it works. Can you let us know the release/SP you are on, and QA will see if it can be replicated. If you are on the release/SP above, it would be very odd indeed and you can file that SR.