Hi,
For now the Python API for "building" Compositing nodes (or node groups more precisely) misses a Python-scriptable node a bit as the Blender Game Engine had: one node, one linked python file in the Text editor for example. This Python node should be able to take namely at least an image input socket, and output and image socket.
For me, this is blocking a bit an easy integration of the G'MIC Python/C++ binding which provides more that 500 hundred 2d filters.
The Blender Custom Nodes stalled Github project by @bitsawer had introduced within a probably easily splittable C/C++ patch:

• a Python-scriptable compositing node (with images input and output)
• a G'MIC compositing node
• a GLSL compositing node

If the Python-scriptable compositing node could extracted from that patch, adapted and implemented, I could then more easily implement a pure-python G'MIC compositing node with the G'MIC Python binding for which I have been working full time for 8 months. Also many people would be interested in writing numpy-based compositing node scripts I am sure.
This was thought over on this Github Issue on the now sleeping blender-custom-nodes project. :-)