This is a somewhat delayed reply and I would like to apologise for that.
Thank you all for your very helpful suggestions. I have solved the problem - and in more than one way.
Using WKWebView, as per the suggestion above, I was able to run SaxonCE and pass out the results via a callback handler.
I also managed to work out the correct URL construction to use a 'requests' POST to the oxgarage site.
Finally, I used the WKWebView python to generate book covers automatically and take JPEG screenshots of them to be used in the EPUB books. Please see the linked images of an example EPUB book I produced using pythonista.
Once again, thank you for your assistance.
Images: