Override a single JavaScript file from a module
The answer is simply that the libraries-override definition must go in the .info.yml not .libraries.yml:

So in mythemename.info.yml:

libraries:
  - mythemename/global-styling
libraries-override:
  toolbar/toolbar:
    js:
      js/views/BodyVisualView.js: js/views/BodyVisualView.js


If placed logically there, alongside the libraries definition, it works perfectly.

Note that the second 'toolbar' is the library name from toolbar.libraries.yml, other library names in there include toolbar.menu, so if overriding a file from there it would be "toolbar/toolbar.menu:" instead of "toolbar/toolbar:".