Hi,
I've found a possible workaround , but i'm not sure that is the best solution.
1) extract ESRI.ArcGIS.Mapping.Core from your Viewer.xap
2) add it in your Add-ins project reference
3) using MapApplication.Current.FindObjectInLayout("myGrid") get your element that have "ViewerApplication" as DataContext
4) ViewerApplication app = (myGrid).DataContext as ViewerApplicaton
5) And now it's possible to retrive GeometryService url for example. (app.GeometryService)
M.