Not sure how it is here, but I know that in some places the no trespassing signs are the farmers way of trying to protect himself from being sued if you injure yourself on his land. Since there are public rights of way through many farms (wanderwegs) then I think there should not be a problem.

As for places to camp... One spot I have my eye on to spend a night this summer is in the saddle between Piz Nair and Suvretta, in Engadin (over St. Moritz and Silvaplana), there is a little lake there and you are well away from it all. I've only been up there during the day but it seems like a really quiet place to spend a night.