In the presence of transaction fees, I think we can rule out many sources of 'noise' in measuring the velocity of money.  One can move coins to a different account within a wallet without incurring fees, and if wallets become sophisticated enough for accounting purposes, a natural desire to avoid incurring expenses ought to mean that money hardly ever moves between wallets unless it is genuinely being moved between actors. 

So we should be able to know, instantly, what the velocity of money is.  We can detect how much of the money supply is spent in every ten-minute block and we can react.