Is starting a jet engine without ground support possible?
Modern jet transport category aircraft have APUs and they will be running if no engines are.  Batteries on jets do not work very long with all electrical buses energized and the first thing you do after turning the batteries on is seeing that they are working correctly and then starting the APU (assuming no GPU is hooked up, if there is a GPU then starting the APU will be deferred until just before the GPU is disconnected).

Thus, any aircraft on a theoretical no-engine taxi to the runway will have a running APU providing electricity to run the airplane and bleed air for an attempt at air conditioning (some aircraft are better at this then others).  When it is time to start the engines they can be started using the APU as the bleed air source and this is a normal method of starting and no different than the method used currently when the plane leaves the gate (and for some airplanes they only start one engine and taxi on that as long as possible before starting the other engine).

As long as the plane has a running APU, it can complete a normal engine start with no ground assistance.  The only limitation will be turbine warm up time, which just means you'll need to make sure the engines are started a few minutes before takeoff thrust is applied.

Just an aside, but currently the only ground support strictly required is a tug to push the airplane away from the gate and someone to operate the jetbridge.  Aircraft and engine startup is self-contained unless the APU is inoperative.