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.
How to prevent the HTML editor appearing for certain pages? (Developer question)
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