Bug, this is a great idea - i wish you every success.
Have you considered a simple counterbalance system, to keep hydraulics simple for a good movement range? As collective is lifted up to "light on the skids" the counterweight is released in a controlled manner. You could make mechanism a seesaw, with vertical linkages designed to keep counterweight and heli CG same distance from spherical bush pivot. This gives a very natural movement.
Kev, i like that idea but would have thought operating outdoors would be realistic enough. I guess the next step is full virtual sim, with full motion control. The National Fluid and Power Centre, at Worksop (Notts) have a great deal of experience offering advice for these sorts of systems...