I bought a step-up ring so that I could use my B&W 10 stop ND filter on the Lumix 14-140mm, and gave the set-up a try this afternoon.
This picture is an HDR blend of 3 exposures of 20, 40 and 60 seconds each. Then fiddled with in Photoshop to boost contrast and saturation.
Again, the GH2 did a great job. One of the advantages of the camera is that I got away with using my lightweight traveller tripod, despite the wind.