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.
Not entirely convinced by this. Think of all of the transactions generated by a pool. Given even a tiny bit of concern about the continued existence of the pool, people are prepared to incur the transaction fee in order to remove the risk of the pool disappearing with all of their earnings. And in any case, it's not clear that the distortions generated by transaction fees aren't so high as to remove any benefits from e.g. NGDP targetting.
And in any case, even if we knew all transactions corresponded to an exchange of goods, we still wouldn't know which of these were intermediate transactions and which were final. Only the latter is relevant to the calculation of NGDP and/or velocity. We need to know the value added by each transaction, not the final sale price.