Okay, the best I can come up with so far is intercepting the keydown event in the code behind:
private void RichTextBox_KeyDown(object sender, KeyEventArgs e)
{
if (e.Key != Key.Tab ||
(Keyboard.Modifiers & ModifierKeys.Control) != ModifierKeys.Control)
return;
var richTextBox = sender as RichTextBox;
if (richTextBox == null) return;
if (richTextBox.Selection.Text != string.Empty)
richTextBox.Selection.Text = string.Empty;
var caretPosition = richTextBox.CaretPosition.GetPositionAtOffset(0,
LogicalDirection.Forward);
richTextBox.CaretPosition.InsertTextInRun("\t");
richTextBox.CaretPosition = caretPosition;
e.Handled = true;
}