So the current Discourse-on-Ghost integration would only need the D-o-G functionality to be self-hosted and not necessarily Ghost. Since that integration uses DiscourseConnect, it will work fine with any Discourse install out of the box, regardless on how it’s hosted.
Which would require self-hosting Discourse. So what problem would it solve? The burden would shift from self hosting a relatively simple Ghost-add on, to self-hosting Discourse.