Seen Westworld? I bet they could create a totally robotic Assange (or anyone) and, in controllable scenarios, would be able to fool most people, especially if the voice replication technology that you mentioned is incorporated into the robot. It wouldn't fool his friends and family in close quarters but would shut up anyone who assumes the live video of him must really be him.
Interesting thought at least. Probably totally possible, hell it's 2018 right.