The Windows Installer team released Windows Installer 4.5 Beta 2 recently. While not a lot has visibly changed since the first beta for which I provided an overview, it's important to note that a new column was added to the CustomAction table since changes to column types are not supported in a transform or patch but adding a new column is supported.

The ExtendedType column is defined as a nullable DoubleInteger, or I4 using IDT format codes. Currently the only supported value is the new msidbCustomActionTypePatchUninstall (0x8000) that denotes a patch uninstall custom action, which is the feature to deploy a custom action in a patch that will execute when that patch is uninstalled. This was previously impossible without hooks in a baseline product because a patch is uninstalled by removing the patch from the aggregate view of the product and all applicable updates, and then repairing the product.

For more information and to sign up for the Windows Installer 4.5 pre-release program, visit Windows Installer 4.5 on Microsoft Connect.