string escape into XML-Attribute

Modifying the solution you referenced, how about

public static string XmlEscape(string unescaped)
{
    XmlDocument doc = new XmlDocument();
    var node = doc.CreateAttribute("foo");
    node.InnerText = unescaped;
    return node.InnerXml;
}

All I did was change CreateElement() to CreateAttribute(). The attribute node type does have InnerText and InnerXml properties.

I don't have the environment to test this in, but I'd be curious to know if it works.

Update: Or more simply, use SecurityElement.Escape() as suggested in another answer to the question you linked to. This will escape quotation marks, so it's suitable for using for attribute text.

Update 2: Please note that carriage returns and line feeds do not need to be escaped in an attribute value, in order for the XML to be well-formed. If you want them to be escaped for other reasons, you can do it using String.replace(), e.g.

SecurityElement.Escape(unescaped).Replace("\r", "
").Replace("\n", "
");

or

return node.InnerXml.Replace("\r", "
").Replace("\n", "
");