Many thanks for your response PIBa...
Since posting my question I have learned that re-writing the headers, using actions in HAProxy is not going to help rewrite the content of the pages themselves (my bad, I'm a noob). The calls to the MongoDB were not direct as you suggested, they were indirect responses from the parseserver app. The solution has been to simply tell the db to declare these https:// urls and that works fine. I had assumed if the db and the parse server weren't actually TLS themselves this might not work, but that was over thinking it perhaps.