There's three options currently available.
Independent throttles can be used on EU specified machines, but only to four mph. How useful this would be to help from a standing start depends how the motor controller is set up. In the case of the Panasonic 'walk-alongside' throttle, it is so low powered it cannot be considered for use while the rider is seated on the machine. At least one Wisper model, the 705, when configured for EU use the throttle works independently, but only to four mph. The difference is, full power is available which means it can help to get the machine moving, even on a steepish upslope with the rider seated.
Centre-drive machines such as Panasonic, Yamaha, Bosch, Daum and many other new systems are all fitted with instant response pedal torque sensors incorporated into the crank-axles. The only time any of these machines would benefit from an independent throttle, is when climbing a hill too steep to ride. As mentioned in the first paragraph, the Panasonic throttle will at least provide enough power to help when pushing the machine up such a hill.
The currently popular Tonaro machines, which do not use crank-axle torque sensor, could be fitted with a chain tension sensor such as the
TMM4. Sadly, losing the throttle on the Tonaro machines would deprive the riders of a unique capability shared only with Cyclone powered bikes, anybody figured out what that is?. Here's a couple of clues. Independent throttle and Deraillure gears. One more clue, 0-15 mph from a standing start.
EDIT: The TMM4 chain tension sensor could be used on hub-motored machines, the TMM4 signal fed into the controller throttle input line.
All the best
Bob