I can work with Sibelius to write sheet music.
From your project description, I conclude that you need your freelancer to combine your base files, continuo part, and obbligato instruments, am I right? If it so, maybe I can help you, but I don't really understand with your statement about copyrights, where should I look for the source? On my own, IMSLP, or you already have the source?
I'm sorry if I misunderstand your need, you can just ignore me then :)
Thank you for your consideration