This won’t solve your problem, but this is something that a few sites that publish WordPress posts to Discourse with the WP Discourse plugin wanted to accomplish. For that case, the WordPress plugin was able to provide a workaround.
To allow this to work with posts created with the Javascript embed code would require a new feature to be added to the core Discourse code. It might be a very useful feature for sites that struggle to get their external posts accurately displayed in Discourse topics.