MLB Trade Rumors profiles Carlos Pena. I wouldn't be opposed to the Pirates acquiring Pena, who I think is probably underrated at this point due to his very low batting average. He does everything else well - he hits for good power, walks a lot, and fields decently. He could also probably be had for very little prospect value, due to his salary.
The problem is that he's left-handed. If the Pirates are going to upgrade at first or in right, a righty would be better. Jose Tabata really should play every day, or most days, when he returns (you can't very well turn him into just the right-handed side of a platoon), and if the Pirates were to acquire Pens, that would leave one position for Garrett Jones and Alex Presley.
As good as Presley has been so far, I'm not yet sure he's not a part-time player in the end, and I don't think the Pirates should necessarily wait for his hot streak to end before making a move that would reduce his playing time. They might get left out in the cold if that happens. If the Pirates acquired Pena, I'd send out Tabata, Jones and Pena against most righties, and use Presley as a fourth outfielder who plays a lot. I think that's an upgrade, and it would give the Pirates some good pinch-hitting options.
That said, if the Pirates could just acquire a righty instead, that would be easier. The guy on the Pirates' bench who's supposed to hit lefties, Matt Diaz, hasn't really done it, which makes him a bit of a square peg. Ryan Ludwick has hit lefties very well this season (.276/.337/.447 while playing half his games in PETCO) and could probably also be had without giving up many prospects. It wouldn't be a sexy acquisition, but I think it would be an effective one. Josh Willingham might be another possibility, except he's a pretty brutal defender and really only plays left field, which is a bad combination in PNC.
If the Pirates acquired Ludwick, they could start Jones at first and Presley in the outfield against righties, and Steve Pearce at first (when he returns) and Ludwick against lefties.