Hi Jim,
I remember many years ago operating then a very old lathe with a flat belt drive that had a clutch consisting of two pulleys side by side mounted on a shaft. One of the pulleys was fixed to the shaft the other was loose. By moving the shaft sideways the belt moved from turning the loose pulley onto the fixed pulley so engaging the drive. I think the gear drive for the head stock was on a spline so not effected by the movement of the shaft. Whether a similar system could be used with a tight round belt would work I don't know. The flat belt using this simple system being a little slack. A interesting problem.
Take care Tony.