Why is CKEditor refusing my custom tag?

With a custom CKEditor plugin I am trying to insert a custom HTML tag, but the tag gets removed as soon as it is inserted into the editor. My plugin.js file:

CKEDITOR.plugins.add( 'tweet', {
    icons: 'tweet',
    init: function( editor ) {
        editor.addCommand( 'insertTweet', {
            allowedContent: 'tweet[:id]',
            requiredContent: 'tweet',
            exec: function( editor ) {
                console.log('inserting');  
                editor.insertHtml( '<tweet :id="\'123\'" />' );  // also tried <tweet />
                // editor.insertHtml( '[tweet :id="\'123\'" /]' ); // this works
            }
        });

        editor.ui.addButton( 'tweet', {
            label: 'Insert tweet',
            command: 'insertTweet',
            toolbar: 'insert,0'
        });
    }
});

The way I am adding the plugin for Bolt CMS backend:

function run() {
    var extrasAdded = false;

    if (typeof(CKEDITOR) == 'undefined') return;

    CKEDITOR.plugins.addExternal('tweet', '/assets/plugins/tweet/plugin.js', '');

    CKEDITOR.on('instanceReady', function (event, instance) {

        if (extrasAdded === true) return;

        var config = event.editor.config;

        config.toolbar.push(
            { name: 'insert', items: [ 'tweet' ] }
        );
        config.extraPlugins += (config.extraPlugins ? ',' : '') + 'tweet';
        config.extraAllowedContent = 'tweet';  // also tried 'tweet[:id]'

        CKEDITOR.instances['body'].destroy();
        CKEDITOR.replace('body', config);

        extrasAdded = true;
    });
}

if (document.readyState!='loading') run();
else document.addEventListener('DOMContentLoaded', run);

Can someone smart see why my tag is rejected?

So it turns out that we don't need the allowedContent or requiredContent properties in the plugin.js script. What did the trick was to tweak the editor's HTML DTD rules. In my case I got a reference to the editor in the instanceReady callback and tweeked it like this:

// name = 'tweet'
editor.filter.allow(name + "[!*]", name, true);

CKEDITOR.dtd[name] = CKEDITOR.dtd;
CKEDITOR.dtd.$empty[name] = 1; // allow self-closing tag
CKEDITOR.dtd.$blockLimit[name] = 1;
CKEDITOR.dtd.$nonEditable[name] = 1;
CKEDITOR.dtd.$object[name] = 1;
CKEDITOR.dtd.$inline[name] = 1; // $block won't work!

You can also see a full gist of it.