Slackening the belt, to use it as a clutch, is not unusual, on all sorts of machinery.
Some time ago, a simple modification, used slackening the belt as a form of clutch on the Warco BH600.
IIRC it was designed and made by Brian MacMahon.
For anyone who wants to look through back numbers of M E W , it could be implemented on any of the clones of the Taiwanese lathes, such as the Chester Craftsman, Engineers Tool Room, BL12 – 243 etc.
Something similar could be applied to almost any lathe using V belt drive, possibly even flat belt drive?
Howard