I agree rocks are just too solid, I've tried and tried and tried. But the experts say it just cannot be done.
Yes, African Rocks (only the scaley type) can be probed. I've had many large boids in the past including rocks, You have to know what your doing (or you can cause some major damage) but it can be done with the right probes.
I'm not sure I've ever heard that they cannot be probed, I wouldn't imagine Mr. Bob Clark stating that, so I can only asume there must have been a misunderstanding (it happens).
Hope this helps
Rusty