python - How can I retrieve private tag GDAL_NODATA from a TIFF using libtiff?

In an earlier post I reported about my attempts to read TIFF with data stored in tiles using python and libtiff. First I tried to use libtiff's function TIFFReadEncodedTile but finally I tried function TIFFReadTile. I have been able to make progress now. The question I have today: how can I retrieve the value of private / custom tag GDAL_NODATA. Both GDAL and ArcGIS make use of this tag. I've seen examples of how to do this in C++ and C#. I still wonder how to do this in Python though. I guess it would be possible by means of the class TIFFExtender and the function add_tags - both defined in module libtiff_ctypes. Could anybody provide an example?