[wix] How to add a WiX custom action that happens only on uninstall (via MSI)?

Here's a set of properties i made that feel more intuitive to use than the built in stuff. The conditions are based off of the truth table supplied above by ahmd0.

<!-- truth table for installer varables (install vs uninstall vs repair vs upgrade) https://stackoverflow.com/a/17608049/1721136 -->
 <SetProperty Id="_INSTALL"   After="FindRelatedProducts" Value="1"><![CDATA[Installed="" AND PREVIOUSVERSIONSINSTALLED=""]]></SetProperty>
 <SetProperty Id="_UNINSTALL" After="FindRelatedProducts" Value="1"><![CDATA[PREVIOUSVERSIONSINSTALLED="" AND REMOVE="ALL"]]></SetProperty>
 <SetProperty Id="_CHANGE"    After="FindRelatedProducts" Value="1"><![CDATA[Installed<>"" AND REINSTALL="" AND PREVIOUSVERSIONSINSTALLED<>"" AND REMOVE=""]]></SetProperty>
 <SetProperty Id="_REPAIR"    After="FindRelatedProducts" Value="1"><![CDATA[REINSTALL<>""]]></SetProperty>
 <SetProperty Id="_UPGRADE"   After="FindRelatedProducts" Value="1"><![CDATA[PREVIOUSVERSIONSINSTALLED<>"" ]]></SetProperty>

Here's some sample usage:

  <Custom Action="CaptureExistingLocalSettingsValues" After="InstallInitialize">NOT _UNINSTALL</Custom>
  <Custom Action="GetConfigXmlToPersistFromCmdLineArgs" After="InstallInitialize">_INSTALL OR _UPGRADE</Custom>
  <Custom Action="ForgetProperties" Before="InstallFinalize">_UNINSTALL OR _UPGRADE</Custom>
  <Custom Action="SetInstallCustomConfigSettingsArgs" Before="InstallCustomConfigSettings">NOT _UNINSTALL</Custom>
  <Custom Action="InstallCustomConfigSettings" Before="InstallFinalize">NOT _UNINSTALL</Custom>

Issues: