There's really no simple and effective way to do this. You have to find the right legend box primitive that matches the pie slice, then find the matching text, which luckily follows the legend box. You can try something like this:

Text textPrimitive = null;

private void ultraChart1_ChartDrawItem(object sender, Infragistics.UltraChart.Shared.Events.ChartDrawItemEventArgs e)
{
   if (e.Primitive == textPrimitive)
   {
      textPrimitive.labelStyle.Font = new Font(FontFamily.GenericSansSerif, 12, FontStyle.Bold);
   }
}

private void ultraChart1_FillSceneGraph(object sender, Infragistics.UltraChart.Shared.Events.FillSceneGraphEventArgs e)
{
   int row = 0;
   double value = 0;
   foreach (Primitive p in e.SceneGraph)
   {
      if (p is Wedge)
      {
         if ((double)p.Value > value)
         {
            row = p.Row;
            value = (double)p.Value;
         }
      }
   }

   foreach (Primitive p in e.SceneGraph)
   {
      if (p.Path!=null && p.Path.ToLower().IndexOf("legend") != -1)
      {
         if (p.Row == row)
         {
            textPrimitive = e.SceneGraph[e.SceneGraph.IndexOf(p) + 1] as Text;
         }
      }
   }
}