quite right, bikes have been racing with that facility for donkeys, and the cost of such system for a road bike is realatively cheap nowadays, but the biggest reason for that is that the cassette gearboxes of motorbikes is already all in order, so even a standard bike can shift gears without the clutch, up and down the cogs, with a momentary shut off and pressure on the shifter going up, or coming down pressure the shifter and give her a quick blip of throttle within the sweet spot range.
going up the cogs, the rev limiter does also serve to aid this too, but its a very harsh methiod of achieving a clutchless change, and not good for your life and licence on the road either!!!
but for a "keep it in" quickshift, the devices used are electronic cuts of igntion and fuel to unload the motor and gearbox, triggered by adequate pressure on the shifter via a sensor on the shift rod,
its simple and good, and more accurate than the average booted foot! but i've never run one, being a suzuki gsxr man, they are the slickest gearboxes out there, and i find i can shift plenty quick enough as they are as a result (been a while now though!
but like i said at the beginning, its all because they are in order of a simplified movement up or down on the shifter, not a H shape through sideways, back and forth motions with joints, slack, and travel to consider.
have you considered some of the already achieved short throw shifter, most folk seem to feel that they have worked well in their sud/sprint/33 and feel more positive and accurate.