After discover that a ghostwriter had listed their work on a Nancy Drew Diaries book on LinkedIn, I decided to see if any Hardy Boys ghostwriters had done the same. It turns out that one did:
https://www.linkedin.com/in/eileen-keller-b42aa5/
"Hot Wheels- Casefile #91
publication date 1993 The Hardy Boys
publication description: Wrote a Hardy Boys book about Solar Cars. Great learning curve and fun projecting possible problems and solutions for this industry."
Despite writing one of the best Hardy Boys Casefiles, Eileen Keller does not appear to have written any more Hardy Boys books.