Stéphane Sudre's site is the Mike Bombich of the packaging world. Scroll down to the Packagemaker how-to for a great overview of the pkg/mpkg formats. This should enlighten on how scripts can be used in packaging.
http://s.sudre.free.fr/Packaging.html
His how-to is to die for (even though it's a little dated, the info is very useful):
http://s.sudre.free.fr/Stuff/PackageMaker_Howto.html
I hope JAMF are keeping an eye on his Packages application...hint, hint. ;)
http://s.sudre.free.fr/Software/Packages/about.html
I'm CC'ing Stephane...hoping when he updates his page that he replaces LANrev InstallEase with JAMF Composer. :):):)
Might want to join the Installer-Dev list, since that's where you'll find some of the most useful conversations regarding packaging. Look for a poster named "Iceberg-Dev"...
http://lists.apple.com/archives/Installer-dev/2009/Nov/msg00042.html
Hope this helps...nice meeting you today at the Dallas user group meeting.
Don