Like Potts, I have pretty much given up trying to contact private tree owners unless we share a 'leaf hint'. The frustrating thing is that the private tree - if reasonably accurate - could hold a key to one of my difficult brick walls. They don't care and I will spend more frustrating years and more dollars looking. In my experience, sharing genealogy brings good karma. I share or help someone and someone, often out of the blue, shares something valuable with me.
You can mark everyone born after 1900 as 'living' so the information will remain private. Since the lion's share of the matches happen prior to that, the useful information can still be shared.