The reasoning is dead simple, and you can have what you want already.I wouldn't have it work on the pedals for some of the reasons above. Though if I were building such a system, best way I could see would be through hall effect sensors and a magnet with it's pole flipped for specific pedal angles. Have that run to a microcontroller that sends a voltage to the throttle input and if you backpedal one full revolution into that position (so you'd need 3 hall sensors, one with a reversed pole, unless N/S sensing sensors are a thing), microcontroller counts the number of pulses, sees that the magnet has an opposing pole to what is normal and locks the bike to the current speed that it is going. You could even eliminate the need for opposing poles and have the bike speed locked after 2 full backpedal revolutions. Maybe allow for some additional backpedalling so you can get your pedals lined up for corners, one full revolution forward or back before it disengages. Probably only good for road use though.
Doubt it would be legal though, the laws need relaxed further and it's a pity (but wholly unsurprising) that the recent consultation got shot down. The UK (and other European nations) are dead set on banning everything into 'safety' and have a rather myopic view that, for example, if deaths caused by throttle powered ebikes are now higher than pre-2016, it must be the throttle doing it and not the fact that many people who are already breaking the law, are going to do so in other ways. Given that I've tested a throttle and a bad pedal assist system (which many people may end up getting on entry level ebike kits) I can say I'd never use that pedal assist because I don't want my bike launching me into traffic while I'm turning the pedals back to I can set off at a reasonable speed from lights, I can however, trust a throttle. As I've already said, someone in government was probably handed an extremely expensive ebike and told "This is how pedal assist works" and they voted on having that be the only way an ebike can function, moreover it was likely an MEP, so a person who isn't directly elected. Though in a twist of irony, the UK would have probably never have made EAPCs legal had we left the EU by then.
A course of action (not the best one, the best would be simple deregulation up to a point like they have in the US) I could see would be to make throttles and higher powered motors legal, with a caveat. Simply do a one or two hour course on "How to use an advanced ebike", no need for insurance, no need for a registration number plate on your bike, just a little "If bike is going to crash, take hand/thumb off throttle" card in your pocket.
Want to make it even safer, there's a system used on boats that is cheap, easy to fit and not bulky; a strap which you put around your wrist attached to the engine kill. All it would need to do on an ebike is close a switch (or bridge a circuit) that allows the motor to function.
That could even be combined so that riders without certification would be required to have to have a safety tether, people without it would be recommended to use one, you could even throw it in as a freebie for doing the £50-75 'course' you do, given in bulk a wrist strap and a switch costs next to nothing.
An EAPC is a bicycle, and a bicycle needs the rider to pedal. That's fundamental to the definition: 'Electrically Assisted Pedal Cycle'.
You want a very low powered electric motorbike. Well you can have one, but it then becomes in law an LPM: 'Low Power Moped'. And it needs to go through a one time approval process.