Because we're using a customized version of FilesInUse, we should disable the MSI version of the dialog in all installers.

Here is some information on how this will be accomplished:

How do I prevent the FilesInUse Dialog from displaying?

A FilesInUse dialog box alerts the user of processes currently running files that must be overwritten or deleted by the installation. This gives the user the opportunity to shut down these processes and avoid having to reboot the computer to complete the replacement or deletion of these files. If you know that shutting down the application is not feasible, or if your setup requires a reboot at the end anyway for some other reason, you may want to prevent this dialog from being displayed.

To do this, set the Attributes column of the Dialog table for the FilesInUse dialog to 0.

Depending on your authoring tool, this is equivalent to marking the dialog to be "hidden", "modeless", and "NoMinimize".

Note that this trick will only work if your setup runs with full or reduced user interface. In basic UI mode Windows Installer uses its own built-in dialogs instead of the dialogs you authored in the msi file.