Eivind, 

Have you tried just using the _ContextMixin?

define([    
    "dojo/when",
    "epi-addon-tinymce/tinymce-loader",    
    "epi/shell/_ContextMixin"    
], function (    
    when,
    tinymce,
    _ContextMixin,
) {

    tinymce.PluginManager.add("custom-plugin", function (editor) {
        var context = new _ContextMixin();

        when(context.getCurrentContext()).then(function (content) {
            //TODO: custom code
        });
        
        return {
            getMetadata: function () {
                return {
                    name: "custom plugin",
                    url: "https://www.custom.com"
                };
            }
        };
    });
});