What you've described is exactly how the F1 MGU-H works (as I posted below). F1 is getting rid of it next year because it costs too much for an F1 team which should tell you a little bit about the complexity involved.
GM is doing nothing of the sort, it's just an ECU map. Guessing here, but if driver presses on the gas quickly (throttle accel > some set value), juice the engine map to create extra exhaust pressure to spool up the turbo impeller. It's all software.
In theory (though not a mechanic, just have an interest in this), the best middle-ground would be to retain the same design but add a motor to the turbocharger shaft which would mainly be reserved for spool ups -- is that what they're doing?