Default:

Desired:

Workaround: handle the ExplorerControl.MainNavigationTreeView.NodeFormatting event and replace the node's text with the FileBrowserTreeNode.Label: 

    Sub New()

        InitializeComponent() 

        AddHandler Me.RadOpenFileDialog1.OpenFileDialogForm.ExplorerControl.MainNavigationTreeView.NodeFormatting, AddressOf MainNavigationTreeView_NodeFormatting
        Me.RadOpenFileDialog1.ShowDialog() 
    End Sub

    Private Sub MainNavigationTreeView_NodeFormatting(sender As Object, e As Telerik.WinControls.UI.TreeNodeFormattingEventArgs)
        If e.Node.Text = "K:" Then
            Dim f As FileBrowserTreeNode = TryCast(e.Node, FileBrowserTreeNode)
            If f IsNot Nothing Then
                e.Node.Text = f.DataItem.Label
            End If 
        End If 
    End Sub