I don't think Castillo offers anything in terms of defense, so SS is probably out of the question, but..... I think this move would give the sox a switch hitting higher OBP guy off the bench who could spell Pedroia at 2B, and what I haven't seen anyone mention is spell Scutaro at SS by having Pedroia get a few innings in at SS with Castillo at 2B. Obviously that's not a long term solution if scutaro is injured, but a few days over the course of the year won't kill us. He would essentially be a MI bench guy but only play one position. He wouldn't be used as a LIDR, but then again, this team doesn't need that. Instead, he would be a get on base, pitch runner, type off the bench. He wouldn't offer any pop, but that's what the 4th OF could be used for (in terms of pitch hitting). If Ortiz struggles, he could also be used in a platoon at DH, especially if Beltre and Cameron struggle with lower OBPs.
Maybe this is just crazy talk, but to me sounds better having Castillo off the bench in this type of role then Huellet. Bench would be Tatis, Castillo, V-tek, and Hermida (or Huellet)