I consider it highly unlikely Patrick Stewart would come back for a quick cameo in a predominantly TOS movie. Riker and Titan might work, but in the end it all depends exactly what time period the 'old spock' parts are set in. We assume it's post-tng but how far? I look forward to finding out when I watch the film.
"Are you sure you won't change your mind?"
"Is there something wrong with the one I have?" - Star Trek IV