Ларчик просто открывался...

<Product>
  ...
  <InstallExecuteSequence>
    <Custom Action="MyAction1" After="InstallFinalize">NOT Installed</Custom>
    <Custom Action="MyAction2" Before="RemoveFiles">Installed</Custom>
  </InstallExecuteSequence>
  ...
</Product>

Т.е. не та секция. Нужна не <InstallUISequence>, а <InstallExecuteSequence>