I have an occasional leak from a pipe in my cellar that appears connected to the wall-mounted toilet on the second floor. Because this is a post and beam house, the bathrooms are stacked and this particular pipe appears to actually run through a beam near the ceiling of the first floor--sometimes the leak appears in this ceiling but not always. The leak occurs in the cellar every 2-4 days.

Although this is an old 5 gallon toilet, there is no sign of water nor leakage anywhere near it, and repeated flushing does not reproduce the problem. My plumber is stymied; he doesn't want to pull this heavy toilet off the wall unless there is good reason to believe the problem is a failed gasket (and he thinks there would be some sign of water by toilet if that is the case) OR that the problem lies directly behind that wall. Another possibility he offers is to open the ceiling between the first and second floors by the beam where the pipes must pass to check for a leak inside.

Any suggestions on how to proceed OR how to better diagnose the problem?

My best advice is to use some type of listening device, such as a stethoscope the next time you see this leakage. Listen for a dripping sound that you can't see. If the leak is running out of the toilet then it will bead and run along the wall, producing no sound. If you can hear a drip then it is probably leaking from the water pipe that supply's the toilet.

I would pull the toilet. This way you will know for sure if seal has been leaking, or at least a good plumber will know at frist sight. This, in my opinion, would be the most quick, easiest place to start. Good starting point. Once the toilet is removed, you can even remove some of the sheetrock behind the toilet and take a look around for clues toward a leaky area. Next step would be what your plumber suggested, tearing into the ceiling where the leak is showing its presence.