How to write a custom action to launch an exe when UAC is cancelledor timed-out during uninstall?

We are building an installer for our product using the WIX technology.
We set InstallPrivileges to ‘elevated’ as we need to write to the HKLM
hive etc.

The application has an exe that is launched after installation and
runs on the system at startup. For several reasons, it needs to be
stopped before the files and executables are removed.

On Vista and Windows 7, we need to recover and launch the exe again if
the user cancels the UAC prompt or it times-out and the application is
not uninstalled.

In the wxs file, we define a rollback action:

<CustomAction Id="RestartApp" Execute="rollback" FileKey="myapp.exe"
ExeCommand="" Return="ignore" />
<InstallExecuteSequence>
…
<Custom Action='RestartApp' After='InstallInitialize' >(REMOVE~="ALL"
OR MaintenanceMode="Remove") AND Installed</Custom>
</InstallExecuteSequence>

However, this never gets called!

How can I detect the UAC is cancelled and recover and launch the exe?