In this thread http://community.infragistics.com/forums/p/67703/343523.aspx#343523
I asked how to rebuild a xamdatagrid - thank you for helping me make progress on that.
As discussed in the thread above we are de-serializing values saved by the user and attempting to restore a XamDataGrid to a prior state.

So now I am able to get a reference to the XamComboEditor in my grid, however the ComboBox property is null and I do not understand why.  Here is the XAML where the XamComboEditor is defined:

<Style x:Key="ParameterScopeStyle" TargetType="{x:Type igEditors:XamComboEditor}">
 <Setter Property="ItemsProvider">
  <Setter.Value>
   <igEditors:ComboBoxItemsProvider>
    <igEditors:ComboBoxItemsProvider.Items>
     <igEditors:ComboBoxDataItem DisplayText="PIMCO" Value="Pimco"/>
     <igEditors:ComboBoxDataItem DisplayText="Account" Value="Account"/>
     <igEditors:ComboBoxDataItem DisplayText="PM" Value="PM"/>
    </igEditors:ComboBoxItemsProvider.Items>
   </igEditors:ComboBoxItemsProvider>

  </Setter.Value>
 </Setter>
 <Setter Property="Value" Value="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type igDP:CellValuePresenter}}, Path=Value}"></Setter>
 <EventSetter Event="SelectedItemChanged" Handler="ParameterScopeChanged"/>
</Style>

<igDP:Field Name="Parameter">
 <igDP:Field.Settings>
  <igDP:FieldSettings EditorStyle="{StaticResource ParameterScopeStyle}"/>
 </igDP:Field.Settings>
</igDP:Field>

I am handling the DataRecordPresenter Loaded event here:

private void CashParameterGrid_DataRecordPresenter_Loaded(object sender, RoutedEventArgs e)
{
 DataRecordPresenter presenter = sender as DataRecordPresenter;
 
 if (! presenter.Record.IsDataRecord)
  return;

 var cell = (presenter.Record as DataRecord).Cells[0];
 CellValuePresenter cvp = CellValuePresenter.FromCell(cell);
 
 if (cvp == null)
  return;

 XamComboEditor editor = cvp.Editor as XamComboEditor;
 ParameterScopeChanged(editor, e);
}

In the code below I need to access the ComboBox property of the passed XamComboEditor however that property is always null:

private void ParameterScopeChanged(object sender, RoutedEventArgs e)
{
 // scopeCombo is a list of data types
 XamComboEditor scopeCombo = ((XamComboEditor)sender);
 DataRecord record = (DataRecord)scopeCombo.DataContext;
 CellValuePresenter cvp = CellValuePresenter.FromCell(record.Cells[1]);

 if (cvp == null || scopeCombo.ComboBox == null)
  return;

 // valueCombo is a list of values of the type specified by scopeCombo
 XamComboEditor valueCombo = (XamComboEditor)Infragistics.Windows.Utilities.GetDescendantFromType(cvp, typeof(XamComboEditor), false);
 ComboBoxItemsProvider provider = null;

 if(valueCombo.ItemsProvider == null)
 {
  provider = new ComboBoxItemsProvider();
  valueCombo.ItemsProvider = provider;
 }
 else
  provider = valueCombo.ItemsProvider;

 provider.DisplayMemberPath = "";
 switch (scopeCombo.ComboBox.SelectedValue.ToString())
 {
  case "PIMCO":
   valueCombo.ItemsProvider = null;
   break;

  case "Account":
   provider.ItemsSource = ViewModel.Accounts;
   break;

  case "Strategy":
   provider.ItemsSource = ViewModel.Strategies;
   break;
  
  case "PM":
   provider.ItemsSource = ViewModel.PM_Names;
   provider.DisplayMemberPath = "Name";
   provider.ValuePath = "Name";
   break;

  default:
   throw new Exception("Scope not recognised: " + scopeCombo.ComboBox.SelectedValue.ToString());
 }

 if (valueCombo.ItemsProvider != null && valueCombo.ItemsProvider.Items.Count > 0)
  valueCombo.SelectedIndex = 0;
}