You could probably use symbolic links. The way you do this is by storing the Data and Cache directory in a different location, and then having 3 different wow folders with 2 symbolic links(Data and Cache dir). This way, each wow copy can use it's own set of AddOns.
You can use a tutorial to make a sandbox environment. The usage is different but the concept is the same:
http://www.nostockui.com/2010/03/mak...rface-sandbox/
There is also a GUI tool to make symbolic links in Vista/Win7:
http://humblecoder.co.uk/?p=50
Wow also doesn't need to be installed in "Program Files", or have any register data. I have kept my wow install on a external HD and I always copy it back when I reinstall my computer(s).