Jason:
It's an interesting question, and bigger than you think. Architects pay liability insurance premiums, and we are good targets for lawsuits, so we have to think about such things from a financial perspective.
We also have to negotiate with clients at the beginning of some jobs as to who has the liability for such things. In my contract with Stone Eagle, I accepted responsibility for turfed areas of the golf course, but not for areas off the turf. I was trying to avoid lawsuits for slip-and-fall lawsuits over stairs that I had little to do with, and for cart paths. I don't know if someone hitting a ball off a rock and back at themselves standing on the grass would be my problem, or the client's. [And the original client no longer exists!]
The rock wall issue is not really much different from the potential to have a ball rebound off a tree and hurt you -- I guess there would be a distinction between BUILDING the wall and NOT CUTTING DOWN the tree.
Incidentally, there was a big lawsuit over Austin CC many years ago. An outsider parked his car by the highway, jumped the property fence, went down to the water hazard by the 3rd or 4th hole [12th or 13th as seen on TV], dove in headfirst, and was paralyzed. The lake had only been dug four feet deep, not eight, and Mr. Dye was sued for negligence. I don't remember if the complainant won his case, or whether it was settled before a verdict, but it was a lot of $. That's why we all have insurance.