For example, getting from this default installer...
...to this one (notice a different icon in the top left, a different text with a different font color, and a significantly smaller size)...
...is really trivial. Go to your platform's harness folder (e.g., if you're building on the 'default' NetBeans Platform, that means that you should go to your installation of NetBeans IDE and then into the 'harness' folder that you find there).
Once you're there, go into the 'nbi' (i.e., "NetBeans Installer") folder and do searches for the icons and strings that you'd like to replace. You might even want to look for the wizard classes that define the installer wizard and you could change those as well:
In other words, I reckon the best solution is to customize those templates, run the "Package as | Installers" menu item, and then see what you'd like to change. Then go back and customize those templates some more. Until you have what you'd like. I'm planning to experiment some more with this, including changing the code in the templates themselves (which are actually NetBeans projects, so that's an interesting thing too), to see how far one can get.
Note: A disadvantage to this approach is if you're creating multiple application on your platform, i.e., the harness is shared between multiple applications. In that case, you'll need to configure, within each application, your usage of the harness/nbi folder. Or, an alternative might be to use different harness/nbi folders for different applications, i.e., copy in a different harness/nbi folder when you create a new installer, which could even be automated via an Ant script.
Any specific areas anyone would like to customize? Try and see how far you get and then blog about it. Curious to know if there's anything that can't be modified in this way.