Yes, there are apparently Sim skins based on Advent Children and other Final Fantasy characters. I discovered this interesting fact when I ran across a video of a Sim AC Sephiroth and Sim AC Cloud making out like mad in Sim fashion, complete with kissy sounds, sexy murmurs, leg twining, and all that jazz.
Or if you prefer, you can see an aggressive Cloud making a move on Sephiroth....
I... I don't know what to say. O_o
EDIT: Yay! YouTube is back up and running!