Yup that’s what I meant!
Yeah I thought that would be the case, but I tried my luck 
Send a hidden payload - yes. It would be wasteful - it depends.
It would be wasteful if there was no way to predict when the frontend changes and you keep sending it. It would be less wasteful to do it when the frontend changes only.
If it’s okay to have a 1 minute delay between each hidden message to Rasa, then this wouldn’t be very heavy (what is 1 message per minute in a conversation, really?).
The least wasteful way is if there is a way to detect when the slots will be needed (through a hidden message from Rasa as I explained here).