Here is some JavaScript code that I used to create a custom return event for a RemoteInvoke:

function OpenImage(url){
   WebImageViewer1.RemoteInvoked = OpenImageCallBack;
   WebImageViewer1.RemoteInvoke('Remote_OpenImage', new Array(url));
}

function OpenImageCallBack(){
   WebImageViewer1.RemoteInvoked = function(){};
   WebThumbnailViewer1.OpenUrl(WebImageViewer1.getReturnValue());
}