I implemented an onerror callback for tempfile.TemporaryDirectory() and confirmed that it cannot be used to unmount a mount point.  Attempting to unmount the mount point from within the callback results in a resource busy error.  It may be related to shutil.rmtree() holding an open file descriptor to the parent directory of the mount point.

I uploaded the program I used for testing on macOS (test-mounted-image.py). The subprocess.run() calls can be modified to run it on other platforms.