Is starting a jet engine without ground support possible? We have had more... than... one question about jet aircraft taxiing about the airport without their jet engines running. Those address several issues such as cost vs savings and regulatory approval.

My question assumes that an in-wheel electric motor, tug or even pull-chain based solution has been developed, certified, deemed cost-effective and installed.

Let's say that I am now sitting on the taxi-way and am 5th in line for take off. It's time to fire up those jets so they can warm up and we can get the beast into the air. According to this question about how turbine engines are started, it appears that I'm going to need some sort of ground-based support to get the N2 stage spinning. That question indicates that a couple of early, WWII era, jet engines had a built in motor that could get the turbines spinning fast enough for a start, but it seems to me that any modern commercial air-carrier grade engine will need some sort of external help to fire up. I suppose if there was a tug that had pulled me to this point, it could have the support infrastructure built in to get the turbines spinning, but any sort of self- (or ground-) propelled system would seem to lack that.

Now that we're nowhere near the terminal, how the heck are we going to start the engines?

I realize that this question is somewhat speculative in nature, but I believe it could be answered based on some existing or in-development ability (that I'm not aware of) to start an engine without ground support. It could also be my misunderstanding that ground support is not required to start a modern commercial aircraft engine (GE90, RR Trent series, LEAP etc.)
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.