It doesn't seem like there is any support for text trimming, since this is generally performed by the .NET graphics object.  Since the Documents engine provides a proxy to this, I tried to work around the issue by drawing the string myself, but it seems that the proxy does not support this format flag.

Your best option would be to try measuring the text yourself through the Documents engine (as mentioned in my reply to your post here); I think that in this case you could simply use the GetCharWidth() method and keep building the string until you run out of room:

string desiredString = "This is some really long text that likely isn't going to fit the header but let's live a little bit because I like to write stuff that goes on for a while";
Infragistics.Documents.Graphics.Font font = new Infragistics.Documents.Graphics.Font("Arial", 14);
ICanvas canvas = section.AddCanvas();
canvas.Font = font;
float ellipsesWidth = canvas.GetCharWidth('.') * 3;
float totalWidth = 0;
StringBuilder currentString = new StringBuilder();
foreach (char c in desiredString)
{
    totalWidth += canvas.GetCharWidth(c);
    if (totalWidth < section.PageSize.Width - ellipsesWidth)
        currentString.Append(c);
    else
    {
        // Remove any trailing spaces at the end before the ellipses
        while (currentString[currentString.Length - 1] == ' ')
            currentString.Remove(currentString.Length - 1, 1);

        currentString.Append("...");
        break;
    }
}

IText text = header.AddText(0, 0);
text.Style.Font = font;
text.AddContent(currentString.ToString());

-Matt