With version 5.1 it’s possible to show the ranking in a dirstyle. Here is a sample how to define a column that shows the rank:

<ColStyle Name="Relevance" Width="80" Height="0" SortName="Rank">
  <CellTemplate>
    <xp:TextBlock VerticalAlignment="Top" HorizontalAlignment="Left" Text="{Binding Path=[folderObject:Rank]}" />
   </CellTemplate>
</ColStyle>