So the answer from the Discourse developers is to “submit a pull request to change the behavior if you believe it can be improved”… which is fair enough really, however, having never written a line of Ruby in my life I’m not best positioned to do this…
Anyone else fancy working out how to prevent Discourse generating HTML that causes mixed content issues? I think it was this pull request which added the feature and it would need a check adding to ensure that a og:image URL in a remote web page, for example: