Create a property that allows to show the UNC name instead of just the drive's letter or change the implementation of the control so this can be achieved by code.

Overrideble properties / methods in DialogViewModels could be added to support custom (or filtered) sources of the navigation TreeView and navigation Breadcrumb.

As a temporary solution, users might filter the breadcrumb this way:
 var breadCrumb = (sender as RadOpenFolderDialog).ChildrenOfType<RadBreadcrumb>().LastOrDefault();
            if (breadCrumb != null)
            {
                RadBreadcrumbBarItem bar = breadCrumb.ItemContainerGenerator.ContainerFromIndex(0) as RadBreadcrumbBarItem;

                DirectoryInfoWrapper bDrive = bar.Items.OfType<DirectoryInfoWrapper>().FirstOrDefault(dir => dir.Name.StartsWith("B"));
                bar.Items.Remove(bDrive);

                DirectoryInfoWrapper rDrive = bar.Items.OfType<DirectoryInfoWrapper>().FirstOrDefault(dir => dir.Name.StartsWith("R"));
                bar.Items.Remove(rDrive);
            }

To filter the treeview:

 var tree = (sender as RadOpenFolderDialog).ChildrenOfType<RadTreeView>().LastOrDefault();
            if (tree != null)
            {
                tree.LoadOnDemand += Tree_LoadOnDemand;
            }
   private void Tree_LoadOnDemand(object sender, Telerik.Windows.RadRoutedEventArgs e)
        {
            RadTreeViewItem clickedItem = e.OriginalSource as RadTreeViewItem;
            List<DirectoryInfoWrapper> dirsToRemove = new List<DirectoryInfoWrapper>();
            foreach (DirectoryInfoWrapper dir in clickedItem.Items.OfType<DirectoryInfoWrapper>())
            {
                // removing drives 'R:' and 'B:' for testing purposes.
                if (dir.Name.StartsWith("R") || dir.Name.StartsWith("B"))
                {
                    dirsToRemove.Add(dir);
                }
            }
            dirsToRemove.ForEach(dir => clickedItem.Items.Remove(dir));
        }

Expose a ShowReadOnly boolean property which will indicate whether the dialog box contains a read-only checkbox and ReadOnlyChecked property indicating whether the read-only check box is selected

FileDialogs are constructed by RadWindow and ExplorerControl as its content. 

Explorer control holds navigation tree, autocomplete, filtering combobox, main pane for viewing fodler and files, search box, breadcrumb. You can check all visual parts in the following help article:

http://docs.telerik.com/devtools/wpf/controls/radfiledialogs/visual-structure

Users might need to put the ExplorerControl in non-window parts of their application (for example in docked RadPane of RadDocking. Also, they might need to override OK/Cancel commands for that purpose.