My theory is that the GH4 was probably the real goal all along, and the GH3 was a stopgap/intermediary product along the way. So it wouldn't surprise me if those 18 pins will do more than power, shutter, two dials, three buttons on top... it doesn't seem like a simple battery grip would really need 18 pins' worth of connectivity. Hopefully I'm wrong though! It would be nice to see alternatives to the YYAARRGGHHH MATEY box.
Not sure it would make sense to send some of the video out the HDMI and some out of the pins. More likely that all the outputs are driven via the HDMI output and the inputs driven through the four pins. Again, i could be wrong.