Playing the rock for images/memory would be little different than Past Visions or Path Tales (and so perfectly reasonable if not particularly interesting).
I think a reasonable interpretation (and one I made when faced with the game situation) would be that rocks and plants have limited awareness. They can't see. They can't hear in the animal sense. They really can only "feel" things like warmth, cold, pressure, wind, wet, dry, pH, etc. A reasonable interpretation is that they have no memories and so once becoming sentient would only be able to report current conditions. So, a rock could only report what it was touching or feeling which would likely be sufficient to determine if the rock touched earth, more rock or air and hence reveal the existence of a secret passage, chamber or small niche behind the rock. In the interest of fun, however, I allowed "memories" of the aforementioned limited awareness. And so the scaling factor allows the caster to tap into the rock or plant's limited awareness and memories of the same sort of telepathically.
But then again, talking trees are the stuff of High Fantasy - so why NOT?
Robin