This may be a silly suggestion, but instead of worrying about tethering the thing, why not just give it some training wheels?
Make a nice light frame out of relatively thin but sturdy metal, and extend the landing gear by 10 feet in all directions. You are then assured that it cannot tip over unless you are greater than 10 feet from the ground. If anything goes wrong whilst in the hover you can cut the power and it will just sit back down.
Ovciously weight is an issue but if you planned it carefully I think it would be fine.