Wav file scale factor
This link implies that .wav files have several standards for storing 32 bit floating point.  It looks like foramt 32-bit 16.8 float (type 1 - 32-bit) would do what you want.  

As best I can see, SOX (reference) can work (read it at least) with this type of format, so you could look at the source code to see how to generate/read these files.  CoolEdit claims to be able read and write this format (see first link.)