"Since it is being a Soft prog the renderer most likely to be Mental Ray, which is superior to Renderman anyways."
You seriously must be mistaken here.
Square definately use Renderman as their renderer of choice. Simply because it's not a raytracer and you have the most control over shaders and rendering parameters. Raytracers are phenominally slow for big scenes and Mental Ray in particular has to tessellate all geometry whereas Renderman can render Patches as patches and absolutely smooth Sub-Ds in record times.
I really won't go into how much better Renderman is for large projects (such as a movie). Sure you can't do raytraced reflections, but you can use env maps or just use frankenrender to connect the rendering pipeline to BMRT.
Easy done!
-Vijay
PS: I've seen several posts of technical directors from Square posting items on renderman newsgroups too.