I pretty much regret to have to re-open this issue. The plugin solution described earlier does exactly what it is supposed to do.
Unfortunately, this only affects the outermost level of the page hierarchy. As soon as I step one level deeper, creating a sub-page of an existing page, the template select box again shows all available templates in a pretty much random order.
This is no longer the question whether this affects the admin user or not. I tried to employ those templates: options in blueprints for the sub-pages, but to no avail. Seems to me that it only gets honored in the *-site.yml files.
Which options do I have to control which templates can and should be used for sub-pages of a particular existing page?