I am afraid that this is not possible. The backend configuration is no longer kept in the app.config/web.config but in the .rlinq file instead. You can, however, use the following constructor of the OpenAccess context class to achieve similar result:
MetadataSource metadataSource = XmlMetadataSource.FromAssemblyResource(
BackendConfiguration config =
EntityDiagramsContext context =
, config, metadataSource);
If you really want to keep those settings in the configuration file, you can try to implement some custom logic that reads the needed settings from there and then use the code above. Hope that helps.
the Telerik team