Union Jubilee Clutch.

[pwdrive]

Ok, here's another, hows about a variable pulley, something along the lines of the old DAF Variomatc, you could arrange for the pulley faces to be moved further apart than the DAF so the belt drops on to a free running collar when the drive power needs to be disengaged, I am not too well up on variable pullies but you might be able to kill two birds with one stone ie, variable speed and de clutch all in one and in the available space.
Nut's or what...

Lawrence.