I have not created an RFE for this. We’re currently updating our packages with a custom pre/postinstall script and other files. To do this, we’re using pkgutil:
pkgutil --expand Vanilla.pkg OutputFolder
We add in our custom files and scripts, then create a package from the OutputFolder:
pkgutil --flatten OutputFolder Updated.pkg
We have python code to do this automatically, but it contains some org specific code. Let me know if you’re interested in a paired down version.
Here’s the trimmed down script. You can supply custom pre/post install scripts, customsettings, brand, and/or actionsite in an ModiefiedFiles directory. They will get placed in the new package: