pkg way to set com.apple.SoftwareUpdate CatalogURL

So I've tried doing this with logGen, pkgGen, and the PackageMaker. I've tried doing it with Composer too. Basically, all that gets done is /Library/Preferences/com.apple.SoftwareUpdate.plist gets updated. From what I'm seeing none of the PKG methods works. It's a simple defaults command for sure. Anyone have a suggestion for best practice on this? Let me know. PJT