I also love both Ghost and Discourse (I have blogs hosted with official Ghost, and have worked for company that uses official Discourse hosted plan) and would love for this to be a more straightforward process too — having Ghost members be the source of truth for Discourse.
Ghost does take note of feature requests from customers so you also could let them know via support@ghost.org
In the meantime, I think you could also possibly post on the marketplace for a developer to set it up one time for you. (As Simon mentioned, if you go with the JSON Web Token authentication option, you could keep both your paid Ghost and Discourse plans, and have another VPS handle Discourse on Ghost)
It would probably be a more suitable request for the Ghost forum marketplace rather than the marketplace here on Discourse. You could potentially also ask / @mention the creator of Discourse on Ghost (Profile - vikaspotluri123 - Ghost Forum) to see if they would be available to do the work.