Using advanced scripts from a dependent library

We have several project, for which I all want to run the same pre and post scripts when building.
Is it possible to store those scripts in a separate, private repository and include that repo as a dependency, and then use the scripts in the platformIO.ini?

You can make a git repository which has the needed scripts plus some additional .ini file, which you can reference using extra_configs. I think standard Python import syntax should also work – see here. Then you can put helper functions into that repo (or larger pieces of logic) that you can import from a little stub script and call functions from it, or execute it directly as an SConscript object (example).