Have you thought about a band brake? I'm sure you've seen one somewhere, they're used on epicyclic gears to actuate them, amongst other things. A steel band with brake lining material runs around a brake drum on the outside, one end of the band is fixed and the other is pulled to tighten the band against the drum. There must be a four or five inch brake drum in the breakers yard somewhere that you could turn the outside smooth to accept a band? The band itself would be quite simple to invent, riveting brake linings backwards on to the band. This would have a gradual release, unlike a "cog" system.
Just my two euro cents,
Matthew
