Ah, I've solved the problem now -- Penelope doesn't like any xml content except its own defined elements.
So then: I have now converted
A Concise Dictionary of Middle English from A.D. 1150 to 1580 by Mayhew and Skeat to the Kobo dictionary format. I used the HTML available at
https://www.gutenberg.org/ebooks/10625 , converted it to Penelope-compliant XML using the Javascript DOM, and then produced the dictionary using Penelope.
I hope that this will be of interest to people.
I have also produced an HTML version of the Canterbury Tales without the interleaved translations (which I find annoying). This is highly suitable for conversion into e-reader formats.
With best wishes,
Peter Day-Milne