It's also possible (and simpler) to do this by making a specific color in the window completely transparent, which causes it to allow click-throughs.
You can see an example of how to do this here, although that was just quick and dirty code, so it needs some cleaning up. I don't even remember if the color which should be transparent is set up as an input there - http://forums.ni.com/t5/LabVIEW/Darren-s-Weekly-Nugget-04-17-2006/m-p/352929?view=by_date_ascending#M179807