What would the call sign of a non-standard aircraft carrying the US President be?
This question is unanswerable in its present state. A glider is a certified and registered aircraft in the US. Ultralights are not register aircraft in the US. And, they are not typically in contact with ATC. Autonomous drone taxis have yet to receive FAA certification, airworthiness, and registration. So, the best manufacturers can do with drones at this time is to fly them unmanned or on test flights.
The bigger question is whether the Secret Service would allow the president to pilot any aircraft. Let alone fly in a non-certified, unregistered aircraft. After all, they do not allow the president to drive a car while he is in office.