After some newsgroup prodding, adding extra dialogs in the form of Notebooks and NotebookPages isn't that difficult, and totally within the bounds of the classes provided to us.
However some extra features could be done for customized Uninstallation dialog support, such as adding a directive or some dynamic functionality to disable the uninstall confirmation dialog (since it would be handled in the manually created "pages"). That is the only big thing I can think of.

Yes of course one could use a simple MsgBox, but having the full range of making a custom page like the installer would be much nicer for the uninstaller.

When I see this feature in other installers, yeah its great. The user gets to see what has open handles/files which the installer thinks it will need, usually a browser or something. Would be nice to see this in IS.

I noticed this a few years back after I installed ISPP and was trying to customize some of the different highlighting. Would be a nice cosmetic feature to have though.