We tried a kind of machinery ring with Central Scotland Smallholders but we couldn't get it to work, partly because of geography(moving the equipment from one place to another was too many miles), the type of equipment (it was too big and unwieldy) and we couldn't work out the financing (insurance, charging, maintenance of the equipment blah, blah, blah). It was just too much work for the small group of volunteers we had.
I still think it's a good idea though and in the right circumstances it might work.
SAOS is a Scottish Agricultural Organisation Society Ltd - which exists to promote co-operation between farmers. If anyone was interested in doing this in Scotland, SAOS might be able to help.