I think this used to be possible, but I’m fairly sure that Discourse now compares the email’s “From Header” to its Return Path. If those don’t match, the email is rejected. (If Discourse isn’t checking the Return Path, the comment system could be easily abused - any email address could be entered into the form.)
There are a few ways that the problem of Discourse as a comment system could be approached. I think the best approach would be for Discourse to improve its comment embed iframe so that users could interact with it as authenticated Discourse users. If that’s not possible, an embedded Discourse comments web app could be developed. That would be an interesting project, but I’d want to be sure that Discourse wasn’t going to provide similar functionality via their embedded comments iframe before pursuing it too far.
There are also some possible WordPress specific solutions. The simplest one being to enable WordPress comments and the WP Discourse plugin. The risk is that this would reduce activity on the Discourse forum. I think there’d be ways of helping with that in the WordPress UI though - for example, link to related conversations that were happening on Discourse.
It would also be possible to develop something specific to WordPress sites that were functioning as the Discourse SSO provider. I wrote about that in previous posts in this topic. To do a good job of it might require significant changes to the WP Discourse plugin. Unless (I’m thinking out loud here):
What I’m trying to indicate with the above screenshot is that for the case of a WordPress site that’s the Discourse SSO provider, comments could be displayed with the Discourse comment iframe. Comments could be created through a form that posts to the Discourse API. This might require some changes to the Discourse comment iframe to ensure that it refreshes when a new comment is added, but wouldn’t require users to be able to interact with it as authenticated Discourse users.