Don't know ANYTHING other than motorbikes other than they have 2 wheels, handlebars and an engine. The rear shocks on y-frame full suspension mountain bikes (the most common type) form one of the 2 links that the rear section of the frame has to the front section. The rear wheel is attached the to back section. The other connection between the 2 frame parts is the bracket near the pedals, which has bearings in to allow free movement (restricted only by the shock).
If I understand Keith here (be a miracle if I have), he's suggesting that at the front of the jocket seat, the seat would be connected to the body of the seat by a bracket with bearings to allow the seat to rotate about that point. You then attach the shock to the back part of the seat, set with about 50% of it's travel used when the driver is sat down.
Yes, the shocks used on higher-end mountain bikes do have lockout switches to lock the shock in position. These tend to be air shocks, which you change the air pressure inside to set the force needed to compress the shock. Some of the more expensive ones also have remote lock-out switches, which you could mount on your console for easy operation.
For one of these shocks, you're probably looking at around £3-500, so definately NOT cheap.
You could build up a workable version with one of the cheap coil shocks, you just wouldn't have the lock-out feature and it probably wouldn't survive quite as long.