The Installer icon could use differently colored arrows to communicate the progress of the installation process.
Multiple simultaneous installations would have to be aggregated.
Icon badges might be better due to color perception differences. Yes, I’m aware of the large percentage of people with color blindness.
As suggested in the comments, the orange pie progress badge would indeed fill as the installation proceeded. The widget is already used in Mail and the forthcoming Safari 3.0 release with 10.5.
Notification methods such as Dock icons and Growl bezels will be all the more useful when virtual desktops spaces are introduced with 10.5.