Why would you want a custom script option before PAR2?
Well for example SRR (rescene), which makes repacking no longer a unpleasantry.
Do par2, then execute SRR and then unpack/delete.
A possible (much easier to implement) feature could be that some of the par2/unpack/deletion methods of python are executable by the script by the outside.
This way you have the same functionality in your script as in SABNZBD, and your script can make the decision what post-processing is needed
What do you think?