Seems to "work" and the website works too.
$ rclone size gcs:test.literature.org
Total objects: 12499
Total size: 80.313 MBytes (84214128 Bytes)
$ rclone size gcs:www.literature.org
Total objects: 12499
Total size: 190.324 MBytes (199568981 Bytes)
Now, when spare time allows later I shall be looking at how normal, real rclone code would allow for any of this - and how to use command line options too. I am thinking that this could well be part of the remote config and not just per-run.