Hello Jens,
Sorry to hear about your experience. Three years ago, I did precisely what you suggest – approach on foot from the HB9 side and activate right next to the border-crossing footpath, avoiding the estate with its horse business. The boundary stone made a good antenna support and allowed me to omit the DL/ prefix (which admittedly confused some chasers) 
Photo taken at 47.650, 8.475
At the time, access to the estate/summit was prohibited anyway due to an occurrence of strangles (horse disease):
73,
Manuel HB9DQM