Got a little further, this is working well as a proof of concept





Changed to a timing pulley drive which certainly helps when braking the motor - the round belt would slip and occasionally jump under heavy loading.
Shortened the threaded shaft and added a 'toothed' assembly above the flywheel which it's designed to engage with. This is disconnected from the threaded shaft and only turns when driven by the flywheel (lots of thrust bearings). I'm yet to connect this clutch part to any loads but when testing it snaps around nice and quickly which is promising (and the flywheel continues to unscrew itself out of the assembly as there isn't a load to stop it..)