Is there a plain-text equivalent to the 'html' suffix for [webform_submission:values]?
Thanks @leymanx for the suggestion to make my own custom token. That's what I did. I created a hook_tokens function that borrowed code from webform.tokens.inc hook_tokens implementation. I deleted anything that didn't relate to the one token I was after, then created a _module_name_token_get_submission_values function that also borrowed heavily from the corresponding function in the webform module. Finally, I wrote a function that performed a series of preg_replace substitutions on the html code that was giving the destination server problems.
Along the way, I learned a lot about custom tokens and was able to refactor my plugin to eliminate the need for a completely separate plugin through the use of a custom token. :-)