Have you tried assigning the electronic zoom to a function key and using the up and down arrows to zoom? I do this and land and it works great (I don't believe the GH2 has this - or at least I haven't found the option). I haven't tried this lens underwater, but thought about doing so using the camera to zoom and building custom gear to engage the manual focus.
I wondered if this was possible and now I see that it is. I bought the zoom gear but have not used it yet, so I can still return it.
Using the zoom gear does seem like it might be slightly more convenient .... Can someone who has used the zoom gear comment ?