Restart compositing from script?

I have a rendering problem which I wanted to solve by writing a script:

So I wrote a script which splits up my scene into N chunks and renders those borders into EXRs. Use compositing is turned off at this point. After that the script generates compositing node group to join everything back to one image so I can apply my own compositing. (If I do compositing on the chunks the merged result will have borders around the chunks of course)

In the nodes I simply use CompositorNodeImage to load the generated EXRs and the output of the group will route to my compositing nodes and that routes to a normal file output with an output PNG.

The question is how to force blender to re-run this newly generated compositing setup?