I can't get config value for 'eoszoom' on my Canon EOS 450D, it's always get me '0'. I see in sources (libgphoto2/camlibs/ptp2/config.c) - yes, it's always zero. Do you plan to implement getting value for this config? I also write program in Windows using ED-SDK, there to get zoom value (and zoom position) you must get property from LiveView image not camera:
err = EdsDownloadEvfImage(camera, evfImage);
// Get the zoom ratio
EdsGetPropertyData(evfImage, kEdsPropID_Evf_Zoom, 0 , sizeof(zoom), &zoom);
In yours API i can't get any properties from LiveView image (gp_file_xxx()).