Hello,
I am a macOS system developer.
I've worked with PackageMaker, Packages and Iceberg, including creating binary plugin bundles (this is when you get a message "The installer needs to make changes" and asks for password upon launch), scripting the installer and dealing with the hidden pitfalls of the Installer.
What the plugin must do? I understand you are building the rest of the installer yourself, and only need the plugin bundle developed, correct?