Creating content with Mercury by overriding save.

gistfile1.js

new Mercury.PageEditor('/path/to/your/controller');
Mercury.PageEditor.prototype.save = function() {
  jQuery.ajax(this.saveUrl, {
    type: 'POST',
    data: {content: jQuery.toJSON(this.serialize()), _method: 'put'},
    success: function() { Mercury.changes = false },
    error: function() { alert("Mercury was unable to save to the url: " + this.saveUrl) }
  });
}