I had to do something like this a few years ago to add a digital signature to a message in XML format. If I remember correctly you have to get your object into a %XML.Document which works in conjunction with %XML.Node. %XML.Node is used to traverse the %XML.Document to get to the section you want in canonical form. Then you pass the Node to ##class(%XML.Writer).Canonicalize(Node) to get the XML as a string which is then passed to the encryption function you use to get your digest/signature. You can pass the whole document or just a subsection to the canonicalize function.
I can't say if it's the only or best way to do it but it was sufficiently quick enough to handle thousands of messages per minute.