I cannot spontaneously think of anything that "es" replaces but the sentence means "it = yes". Consider "Sie (= die Antwort) lautet ja." - "It (= the ansewr) is yes."
I get what you mean, but this context you are talking about is too specific, and I honestly do not understand what could possibly replace 'es' in the sentence. Also, isn't 'is' translated as 'ist' in German? When and why should we use 'lautet'?
The powers that be said they'd accept the entry of 'Yes, it is,'
There's a problem with this language learning method of only having phrases instead of conversations or at least some context. This is one example of that.