I've got a ton of Sony L-series batteries. They're each 7.2V and the BMC can accept between 12 and 30V... so perhaps two could be connected in series...
If that's the case, each battery holds approximately 47Wh, between two batteries that's 94Wh... so this solution would run the camera for at least 5 hours continuously (assuming an average draw of 18W).
Roughly 1.25 Amps for this 14.4V system. The plus of a solution like this, apart from a decent runtime is that it'd be extremely cheap because there'd be no additional electronics and no stepping up of the voltage. This would also bring the efficiency of the system close to 100%, vs whatever losses occur in conversion solutions.
Anyone have any idea where one could find a dual battery mount ?
-edit- how about a cheap generic dual-battery charger for $27
ebay cheap charger ... Just strip out the electronics and solder.