I can't remember if I've seen any reporting definitively on the status of his signing, but I did stumble across this local news piece and interview on YouTube and can't tell what kind of confirmation this would be:
At 1:03, Marcelo's father says their advisor called after the first pick and said they had an agreement with Boston, Marcelo was happy with it, and that the next few picks were very stressful. This would seem to indicate that the Sox have plans to sign both Mayer and Fabian, even if it means going over slot to do so. Apologies if that is a "duh"-level of analysis . I'm not enough of a draftnik to say whether this is obvious to anyone else, or if the remaining picks on day 2 further cement that.