Thanks! The clue was indeed revealed in the "text" resource type. The solution, for anyone else who may need it:
Set the *global* variable $usehtmleditor to 'summary' (i.e. the name(s) of the textarea(s) that you *do* want to be passed through the HTML editor) - if you don't do this, then by default all of the textareas are transformed.
Text editors
How to prevent the HTML editor appearing for certain pages? (Developer question)
Eztabaida hau blokeatu da eta beraz aurrerantzean ezingo duzu bertan erantzun.