Available in LabVIEW 2012 and later. There is a 'Run executable before uninstallation' option on the 'Advanced' page of the Installer build spec dialog.
We currently have an option to "Run executable at the end of installation". This is useful for installing other stuff required by your application. However, we also need the ability to clean up these things during the uninstall process. So, I'd like an option to "Run executable at the beginning of uninstallation".