While I'm sure there are contingency plans associated with being cut off from Air Force One and the need to travel to some rendezvous point to get choppered out, my guess is that the security protocol and planning is designed to prevent that very scenario.
The President doesn't just fly in solo on Air Force 1 -- often they send a second 747 as a backup along with a varying number of C-17s or C-5s with support vehicles and over a hundred Secret Service agents. On a Clinton trip to Africa, the air force ran 24 hour fighter missions overhead and the Navy kept a medical ship offshore.
I'm sure the goal is to both secure the plane, secure the route back to the plane and never leave the President cut off, up to the ability to conduct immediate air strikes to insure he can return to the plane.
There may be operational reliability or contingency issues which would limit an electric car overseas, but I think in DC it wouldn't be an unrealistic option.