@davidfor - I'm afraid that functionality is just way too niche to consider for this plugin. There is such a small range of use cases where it would work. I would suggest the majority of users get their ebooks from a wide range of sources/publishers, which makes it impossible to do such a substitution without doing irreperable harm to the ebook.
I think you would be better off having a bespoke plugin that does specifically just this function. Or possibly even having it as a Sigil plugin (though this is not possible yet, but might be one day if it ever gets a plugin API).
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