I was looking at some director's view finder apps such as Artemis, Viewfinder pro, and Panascout? Certainly much cheaper than a real viewfinder, but I wonder how well it works.
These apps use the internal cam of the phone. They are ment to do easier preplaning of your shots, not to monitor the actual output of your cam. Anyhow, some phones like galaxy S2 support USB host mode. For canon there is already a app that supports monitoring the output of your cam, but from what i red its buggy and slow. Dunno if GH2 offers the option to act as webcam like canon does, but a free high res viewfinder would indeed be great.