This includes some guesswork, so if I'm wrong, hopefully someone will set me straight.
I'm sure from a security perspective, this is much better. But from trying to extend functionality with custom code, it's kind of cut us off at the knees. This is the biggest hurdle for me with the new designer, because if I can't do something with built-in functionality, I always find a way to code it, but that option has been severely limited.
So, bottom-line, I don't think it's possible to do this with the new designer currently.