A little titbit that I picked up whilst speaking to John Rhys-Davies before x-mas and forgot about until I read the piece on him appearing at a comic-con in LA.
I asked JRD about the voicing of Treebeard and he mentioned the difficulties in translating the venerable ent to the big screen (slowness of delivery, long pauses etc.). He also mentioned that PJ and the sound department were experimenting in a ‘layering’ technique in creating the sound of treebeard’s voice.
He spoke of the attempt to integrate the sounds of a tree; creaking branches, straining boughs etc. into JRD’s performance which as you mentioned is yet to be recorded. He took pains to say that this technique was not by any means finalised and that if it didn’t work that they may abandon it.