I think there's no way to customize encoding in xmlwrite and writestruct (from R2020b).
Workaround would be using .NET or Java libraries.
If you're using Windows machine, you can use .NET addAssembly and you can utilize System.XML libraries in MATLAB.
asm = NET.addAssembly('System.XML');
import System.XML.*
% Create XML object
doc = System.Xml.XmlDocument;
% Specify encoding ISO-8859-1
xmldecl = doc.CreateXmlDeclaration('1.0', 'ISO-8859-1', []);
doc.AppendChild(xmldecl);
% Specify DocumentType
doc.XmlResolver= []; % To avoid dtd search errors.
doctype = doc.CreateDocumentType("CAMERA", [], "camera_cal.dtd", []);
doc.AppendChild(doctype);
% Create an element. This is just a sample.
newElem = doc.CreateElement("items");
newNode = doc.CreateNode("element", "item", []);
attr = doc.CreateAttribute("name");
attr.Value = "name1";
newNode.SetAttributeNode(attr);
newNode.InnerText = "text here";
newElem.AppendChild(newNode);
doc.AppendChild(newElem);
% Save the XML object to xml file
settings = System.Xml.XmlWriterSettings();
settings.Indent = true;
writer = System.Xml.XmlWriter.Create("out.xml", settings);
doc.WriteTo(writer);
writer.Close;
Then, you can get the following xml file.
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE CAMERA SYSTEM "camera_cal.dtd">
<items>
<item name="name1">text here</item>
</items>
