Post Gazette has a nice list of the non-roster invitees.
Aside from the prospects (Brault, McGuire, Stallings, Frazier), I think Zarate has a shot at a bullpen spot. He is a LHP.
Stallings is also interesting. I forgot he was another catching prospect. At 26 he is old and has yet to play at AAA, but he could play in AAA this year as a backup to Diaz. He is a big guy at 6' 5".