It costs me some time and nerves to figure this out, but finally ive got this working.

var flowDocument = (FlowDocument)Application.LoadComponent( new Uri( @"MyFlowDocument.xaml", UriKind.Relative ) );

doc.DataContext = this;

var package = Package.Open( new MemoryStream(), FileMode.Create, FileAccess.ReadWrite );
var packUri = new Uri( "pack://temp.xps" );
PackageStore.RemovePackage( packUri );
PackageStore.AddPackage( packUri, package );

var xpsDocument = new XpsDocument( package, CompressionOption.SuperFast, packUri.ToString() );

var paginator = ((IDocumentPaginatorSource)flowDocument).DocumentPaginator;

new XpsSerializationManager( new XpsPackagingPolicy( xpsDocument ), false )
.SaveAsXaml( paginator );

Viewer.Document = xpsDocument.GetFixedDocumentSequence();</code><strong></strong>

The key part here to get the binding to work is this call:

Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.SystemIdle,
new DispatcherOperationCallback(arg => null ), null);

Ive got this tip from the blog of Mark Nijhof. I dont know why this is need, but i assume that it needs one Dispatcher run to expand the DataBinding.