Actually, I think the style of Planescape would mesh very well with FFZ: the planes and factions are perfect vessels for giving form to common Final Fantasy themes, and Sigil's atmosphere is not all that different from Midgard's; I do suppose that mechanics would have to be tweaked so Callers in...
Hmm...I just got an insane idea-using the FFZero system in the Planescape setting. However, the plot I have in mind violates a huge amount of the setting's "canon" so I don't want to talk too much about it. :uhoh: