You probably moved along and found your own solution in the meantime but here is a possible solution. Rather than using the serializer, I directly call the underlying StringWriter to insert raw text (in my case that is well formed xml, so I want to avoid escaping the <,> marks). Notice the serializer flush is very important to get the correct order.
XmlSerializer serializer = Xml.newSerializer();
StringWriter writer = new StringWriter();
try {
serializer.setOutput(writer);
serializer.startDocument("UTF-8", true);
serializer.startTag(namespace, "destination");
serializer.attribute(namespace, "id", String.valueOf(destinationId));
serializer.flush();
writer.write(DestinationContent);
serializer.endTag(namespace, "destination");
...
} catch (Exception e) {
throw new RuntimeException(e);
}