Solution: walk over to a place with equally great scenery and take the photo. Cycling anywhere near the Aso caldera is prohibited (and one can certainly see the wisdom of this), but at least I was able to take the I-was-there photo.

Aso claims to be the largest caldera in the world. This Hawaii boy initially wondered about that claim, but apparently what you see here is only the center of a massive caldera many kilometers in diameter... and in any case it's quite an impressive sight. Unfortunately, they won't let you get anywhere near it with your bicycle, not even to pose for a quick photo. Trying (unsuccessfully) to negotiate with the mid-level bureaucrat at the office was by far the most frustrating part of this trip.