I would be getting on the original developers (which would be Marvelous Entertainment. Aka. the Harvest Moon guys), not Sony. While Sony understandably don't want new PS2 games, it might not be impossible to get them on your side. It's still a long-shot (Marvelous are a pretty big company, and if they had any intention they probably would just do it themselves), but less of a long-shot than trying to get anything out of Sony, who have no vested interest in these games aside from royalties and console sales. The most likely solution would be to do a port, but there are all kinds of issues with that that would make such a thing improbable at best.
Shame really - I've always wanted to play the Gunslinger Girl games, as I am quite a fan of the original anime. I haven't watched much of the second season, and I've heard it's not as good (from what I've seen, this is probably true), but there's enough in there to pique my interests.