SDSS-III does not use the fpC file format for science images. Science images are now released as "frame" files. The primary science image (hdu0) comes background subtracted and calibrated to units of nanomaggies, with the backgrounds and flat-field conversions included as extensions. The astrometric information is in hdu3 instead of a separate asTrans file.
obs_sdss should be able to ingest frame files and map them to load as dataset "raw." It should also optionally replace the backgrounds and de-calibrate to convert the units back from nanomaggies to counts.
This will be implemented as lsst.obs.sdss.sdssMapper.Sdss3Mapper.