Calling coauthors_posts_links(); on the primary blog does not retrieve the guest author, presumably because that guest author is only really stored with respect to the particular blog for which it is a custom post.
Correct. The best approach would probably be to save the guest author data to a global table, and then directly query the global table. You could hook into Co-Authors Plus and add a syncing action that creates or updates the profile data in the global table on save.
Further, I had some odd behavior for a while that must have corrected itself (perhaps it was a migration triggered by calling some specific function?) where guest authors' posts_link did not display, basically since get_author_posts_url() was coming back null
Odd. Let me know if it comes up again.