I'm using a powershell-Action to pop up a MessageBox. The Action is scheduled under OnPackagesConfiguring. The MessageBox is indeed opened - BUT it is hidden by the Wizard-Dialog (Installation Progress). (Note: in my simple test the PS-Action does nothing more than showing a MessageBox)
If you want the parenting of the window to be correct, you have to use UI actions (including methods from ISuiteUIExtension) to show your message. These aren't currently available to PowerShell actions (or managed or InstallScript actions), so there is no parenting information, and then focus stealing prevention can cause the message box to be covered.
