XmlSerializer omit encoding attribute

Is it possible to instruct .NET's XmlSerializer to serialize XML documents to the following format:

<?xml version="1.0"?>
<!DOCTYPE root SYSTEM "root.dtd">
<root>
...
</root>

i.e. XML declaration without the encoding attribute, doctype and then the rest of the document?

Currently I have:

var bomlessUtf8Encoding = new UTF8Encoding(false);
var serializer = new XmlSerializer(typeof(T));
var namespaces = new XmlSerializerNamespaces();
namespaces.Add(string.Empty, string.Empty);

using (var memoryStream = new MemoryStream())
using (var xmlWriter = XmlWriter.Create(memoryStream, new XmlWriterSettings { Indent = false, Encoding = null }))
{
    xmlWriter.WriteDocType(typeof(T).Name, null, ContentSerializer.MlpSvcInit320Dtd, null);
    serializer.Serialize(xmlWriter, input, namespaces);

    return bomlessUtf8Encoding.GetString(memoryStream.ToArray());
}

but .NET is barking when creating the XmlWriterSettings object due to Encoding being null. Is there a canonical way, or is it easiest to simply serialize to a string and then call .Replace() on it?

Unfortunately, a third party vendor requires the serialized string to be exactly in this format.