Both Dickerson and Casilla would have to pass through waivers.
You're right that Valencia is only the DH against LHP, but I think that's a more important role than backup CF on this team, especially since in a pinch McLouth could slide over there. That said, I think it likely may be Valencia because Reimold is back for now and, more importantly, he's got options. If not for the string of lefty starters, I'd say they would definitely be leaning towards him.
If it were me making the decision, it'd be Dickerson. Casilla if I wasn't concerned about Roberts, as you mention.