Private Strip
There is indeed a private strip (grass) on the hill just outside Fort Augustus that is used by the owner of the estate (Glendoe I believe). I don't believe the owner spends much time there and any approach I would suggest would have to be through the estate manager. It is curently used by the owner, I saw 3 aircraft there 3/4 weeks ago and periodically see the owner come & go. Fort Augustus is around 32 miles from Fort William. One current consideration is the construction of the hydro power station in the area, there is currently a workers camp & associated activity adjacent to the strip, but it is still in use.
Of course be aware of the very low flying military, I reckon it won't be much more than 300 feet above the strip.
Let us know how you get on if you do approach them.