Postinstall script in Mac AppStore package created using productbuild

Would it be possible to include a script

No. Mac App Store installers can only install a single app with no customisation along these lines.

and some files in the pkg archive that can be copied into the app's
sandbox after it has been installed using some sort of a post install
hook?

Why not embed those files within the app itself?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@apple.com"

Why not embed those files within the app itself?

Currently those files are within the app. They are basically html files that by default show the content bundled with the app. Users have the option to get extra content from the internet (banners, themes etc) that gets downloaded to the app's container. After upgrading to WKWebView, it seems both the html and resources need to be placed in the same directory hierarchy if the app is sandboxed, otherwise the webpage fails to load local resources. That is why I wanted to place those html files in the app's container after installation so that both the html and resources are in the same directory hierarchy.