Is the basic issue you’re trying to solve that when a user is logged out on WordPress, you want them to be logged out in all WordPress sessions? wp_ajax_destroy_sessions() – Function | Developer.WordPress.org
Edit: I think you could use the logic that’s in the wp_ajax_destroy_sessions function to write your own function that would do the same thing. The WP Discourse plugin should have an action hook added to it that fires right before it calls wp_logout. I can get that added in the next update. You should be able to hook into that action to log users out from all sessions.