Electric meters often blink a signal LED for every X kWh, so other devices can read the signal. I'm not sure if this is used for bidirectional communications, though.
No, because you can install and configure the firewall before you install package X. (without knowing anything about X, your firewall defaults can just prevent X from doing anything)
But you can't (easily) configure package X itself before you install it; and after you install it, it runs immediately so you only get to configure it after the first run.