I'm not aware of ways to accomplish either..

I haven't tried this, but if it's important, maybe you could filter messages from that person (or HTML message in general) and generate new plain text messages that are based on those original matching messages.

To match HTML messages in a filter, I'd try this filter condition: ['FILE' - 'matching mask' - 'Message.html']. As an Action to recreate them in text format, you'd use the filter action 'Create a Formatted Message'. If I recall correctly, it allows you to retain the original header fields and you can use the %TEXT macro in the message template to copy the original text.

(The only other possible solution that I can think of would be to have the filter perform an external action on the messages, in which a 3d party tool -if it exists- would convert the HTML to text).