I think it has been mentioned in How to modify frame buffer returned by pyds.get_nvds_buf_surface? - Intelligent Video Analytics / DeepStream SDK - NVIDIA Developer Forums, there is already NvBufSurface read and write sample in c/c++, you just need to implement in python with gst-python and pyds.
https://lazka.github.io/pgi-docs/#Gst-1.0/classes/Buffer.html#Gst.Buffer
https://docs.nvidia.com/metropolis/deepstream/dev-guide/python-api/PYTHON_API/Methods/methodsdoc.html#nvbufsurfacemap