How can I add an action list to be performed just after showing the "Ready to Install" page but before unpacking files?

You can use the <readyToInstallActionList>, which is executed right before unpacking files:

<project>
    ...
    <readyToInstallActionList>
        <!-- List of actions like uninstalling services,
        environment tweaking, etc -->
    </readyToInstallActionList>
    ...
</project>