Blender has a very good Python interface, no there are 2 ones! Blender module and Blender210 module. However I'm still missing something: A user writable dictionary associated with every obejct that would allow it, to store extended information in it. I'm not talking about game properties! Currently I've not time to add it, however I estimate the afford at about 110 lines:
10 for the python dictonary
50 to extend the blend-file parser and another 50 to extend the writer.
However it could be more or less...

Without having thought it through properly... but why not just make your own wrapper class for the objects? Then add whatever data members you wish to have.