This may be a dumb question, but can you run it on AC from the adapter that comes with it? One more power cable to plug in might not be that big a deal if you're indoors and already setting up lights, external audio mixers and such.
For 6+ hours of running time, you're probably talking about the big Anton Bauer or IDX or Switronix V-mount battery bricks, or a kludge like fabricating a cable to run from a gel cell or car battery. Any of these would require a sturdy voltage regulator to drop the voltage from the 14.4V of the battery packs to the 7.2V that the camera requires. Panasonic has announced, but not yet delivered, an Anton Bauer gold mount battery bracket for the HMC80. I'm not sure when to expect it to arrive. The full system of adapter, charger and one Li-ion battery will cost you around $850 to start in either of these systems. Additional battery packs (e.g. A-B Dionic 90s) are about $400 each. The 7.2V Anton Bauer Elipz system, which mounts between the camera and tripod, might work, but I'm not sure it will quite reach the required run time on a single battery.
