Hi Finski,
I know what you mean, the problem is the land is mostly freehold around the north of Canberra so finding any spot that isn't already owned by someone is likely going to be right next to the Barton (still crown land), which is not necessarily what you want.
There's a few lay-bys in roughly the middle between Murrumbateman and Canberra (too close to the highway) but if you know who owns Spring hill, that might be a good spot. I know that used to be owned by Phil Robinson (not sure if this site is up to date, but look at
this reference) and if you arrange a time and date beforehand he may be amenable to letting you sit on top of the hill for a night.
What a view that would be!