I use lithium packs in my Multiplex Cockpit everyday. The standard Multiplex pack is a 7.2V NiCad/NiMH so it's not a big jump to go with a 2 cell lithium pack at a nominal voltage of 7.4V.

Unfortunately I belive Futaba uses 9.6V packs and the closest lithium pack would be a nominal 11.1V. I'd be interested in seeing what happens if someone were to try this.

Someone could certainly rig up a tiny voltage modulator to fit in the huge void of the 9C's battery compartment with the Etec 3s 1200 pack!