Software Update is an Apple application, and it's only interested in updates to Apple software. I think you'll find that listing is the same as the one in /Library/Logs/Software Update.log
All installer scripts that are based on the Apple Installer create receipts that their own updaters can use to determine what's been installed where, but I don't think there's any standard utility to browse the entire receipts database. Maybe someone has created a 3rd party receipts database viewer?
I would be reluctant to try deleting items in the Receipts folder, given that there's an opaque database file in there that may get upset, or have to be rebuilt at some stage.
Edited by deleted (Mon 30-Mar-09 15:39:24)