Good use case - is it for high-end animation and special effects software... something like Pixar would use?
I guess it depends which parts of the QImage class you want to use... most of the methods take uchar's or QRgb's (which is just a typedef for an unsigned int).
Superficially at least, it looks like a fairly poor fit as an interface to images with floating point pixels. I expect the trolls could come up with something! It's not that mainstream though, so probably not a very high priority - particularly now Nokia owns the company and there seems to be a shift in focus towards embedded products - OpenGL ES doesn't support floating point textures IIRC.
Sounds like a good opportunity to contribute something to Qt! :-)