The modelsupport2.dll is in the TestStandModel directory, but not modelsupport.dll. The functions I need were in the original modelsupport.dll file packaged with TestStand 1.0.2. The functions needed are UIMessage_Event calls.
NI have removed a load of wrapper functions from the modelsupport.dll. The API function PostUIMessage is still available (Thread.PostUIMessage) so it looks like you will have to use the API function or the new PostUIMessageEx function.