It's not supported today, but you can fix it if you change the source and recompile (after all, this is what open source is for...). Go to HtmlNode.cs, find the internal void WriteAttributes(TextWriter outText, bool closing) function, and add the following piece of code:

internal void WriteAttributes(TextWriter outText, bool closing)
{
    if (Name.StartsWith("?"))
    {
        int len = _outerlength - 3 - _namelength;
        if (len > 0)
        {
            outText.Write(OwnerDocument.Text.Substring(_namestartindex + _namelength, len));
            return;
        }
    }
    ....
}