Hello Graham,
To be able to do what you want to do, the editor must support subdirs, I have not checked but I suppose that this is not the case for the editor used in lesson content pages, contrary to other places in Moodle like Label, Page or Book.
Maybe if you are interested in this feature you could create an issue in the tracker, but I don't know if it would be considered as a bug or as an improvement. If the Moodle team consider it is an improvement, even if a developer work on it it will only be available in a future release, it will not be backported to existing releases.
Technically I don't think it would be a lot of work to do this, probably only a small change in the code.
