D2136 Possible Solution

// Usual non-virtual method of IGUIObject, or you may
// split it out and pass GUI* as an argument.
template<typename InterfaceType, typename ObjectType>
JS::PersistentRootedObject IGUIObject::CreateJSObject(
    ObjectType* object, const char* name)
{
    JSContext* cx = GetGUI()->GetScriptInterface()->GetContext();
    JSAutoRequest rq(cx);
    JS::PersistentRootedObject JSObject;

    if (!JSObject.initialized())
    {
        JSObject.init(
            cx, GetGUI()->GetScriptInterface()->CreateCustomObject(name));
        InterfaceType<ObjectType>::RegisterScriptFunctions(cx, JSObject);
        JS_SetPrivate(JSObject.get(), object);
    }

    return JSObject;
}

// IGUIObject interface object
template<typename ObjectType>
class IGUIObjectInterface
{
public:
    static void RegisterScriptFunctions(JSContext* cx, JS::HandleObject obj)
    {
        JS_DefineFunctions(cx, obj, Methods);
        // ...
    }

    static bool JSI_IGUIObject::toString(
        JSContext* cx, uint UNUSED(argc), JS::Value* vp)
    {
        JSAutoRequest rq(cx);
        JS::CallReceiver rec = JS::CallReceiverFromVp(vp);

        JS::RootedObject thisObj(cx, JS_THIS_OBJECT(cx, vp));

        // Here we have a derived/final object type. That means
        // we don't have to do downcast, only upcast. Which is simple.
        ObjectType* object = reinterpret_cast<ObjectType*>(
            JS_GetInstancePrivate(
                cx, thisObj, &JSI_IGUIObject::JSI_class, NULL));
        if (!object)
            return false;

        ScriptInterface::ToJSVal(
                cx, rec.rval(), "[GUIObject: " + object->GetName() + "]");
        return true;
    }

private:
    static JSFunctionSpec Methods;
};

template<typename ObjectType>
JSFunctionSpec IGUIObjectInterface<ObjectType>::Methods = {
    JS_FN("toString", IGUIObjectInterface<ObjectType>::ToString, 0, 0),
    JS_FS_END
};

// Another interface object, it can derive or not, it doesn't matter,
// it also allows to use virtual inheritance.
template<typename ObjectType>
class IGUITextOwnerInterface : public IGUIObjectInterface<ObjectType>
{
public:
    static void RegisterScriptFunctions(JSContext* cx, JS::HandleObject obj)
    {
        IGUIObjectInterface<ObjectType>::RegisterScriptFunctions(cx, obj);
        // ...
    }

private:
    static JSFunctionSpec Methods;
};

// All this objects can be a part of origin GUI objects.