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.