How to present JSON via a e.force:showToast event - escaping of curly brackets?

As far as I can tell, you have to literally use templating to get them to appear correctly:

({
    init: function(component, event, helper) {
        $A.get("e.force:showToast")
        .setParams({title:"Success", 
                    message:"hello world",
                    messageTemplate:"{0}hello world{1}",
                    messageTemplateData: ["{","}"]})
        .fire();
    }
})