Unfortunately, any calls to Isolate.WhenCalled are not officially supported.
what (is( supported however is the ability to create fake instances of of unfakeable objects using the silverunit API - FYI - since silverunit is open source, you should be able to add your own API hooks to silverunit to get the "whenCalled" functionality you want, I think.
to see how, looks at the SilverUnit.MakeEventArgs() method in the source.
hope this helps,
Roy