Southernweyr, not trying to counter your thoughts here about avoiding tethers. My post was not very clear - i'll see if i can tidy it up.
What i meant to say is that the single main problem with tethers is that they apply loads to the airframe which do not pass through the C of G and thus produce a torque high enough to overpower any cyclic input. This will be even worse if the tether suddenly becomes taught as dynamic rolloever is sure to follow. I'm making the assumption here that BugDevHeli wants to find a way to accumulate time and test the machine with minimum risk to pilot. The best method is to have the machine fixed in such a way that the collective cannot alter the machines height, with the simplest being to bolt it to a trolley.
What i was trying to figure out is if there is a method of applying linkages between a mobile ground run trolley and airframe to simulate in flight loads and attitudes but keep machine in check in case of any problems. This would give Bug a feel for the machine as well as expose parts to realistic loads. If there was any problem the machine is still on the ground so there would be no complications.
The only practical way i can think to do this is to use 3 or 4 links all angled in to pass through C of G. This way collective and cyclic can be flown for realistic flight attitude, hence dynamics, hence loads with machine all the while being well supported. The problem though is that if the machine started to fall over (for whatever reason) then the links would now be pointing the wrong side of the C of G and would not stop the machine falling over. A simple solution is to have the virtual apex (where all links intersect) above the C of G, so that machine has to rotate some way before C of G passes outside of virtual pyramid created by linkages. An even safer solution would be to have a sprung centreing mechanism that would overcome any moment as the machine tried to fall over.
Bug, please comment whether any of this is any help. I'll try to clarify more if needed.