Except they do? The stock of money times the transaction velocity divided by the stock of production gives you the price index. The Greek government would definitely know the first, because they're in charge of it, and they know the second, because that only changes very slowly. The tricky part is transaction velocity, but the historical predecedent has been either fairly accurate devaluation pegs, or very rapid adjustment (i.e., an inflation burst lasting a year at most) - certainly not lingering inflation.
EDIT: The whole point was that Argentina STOPPED using the dollar to back the peso. Like, the fact they pegged the peso to the dollar was one of the reasons they had to default in the first place. I mean come on, that's basic research.