Suppose the central bank controls 51% of hashing power and wants to achieve a stimulus equivalent to a 3% increase in inflation.
Simply demand a txn fee equal to 3% of coin-age (with age measured in years). This is just like instantaneously increasing the inflation rate by 3%. Take the fee proceeds and hand them out to banks. Voila. You have stimulated spending. Once the economy recovers, the txn fee can be lowered again. The inflation rate of every single block is completely at the bank's discretion. This is 100% impossible with the tools available today.
Now people might try and horde (the bank can't steal my coins... I'll just wait till they lower the fee and spend then). Ha, this would not work at all. The bank has a historical record of its inflation policy and would bill these guys. The fact that you haven't paid yet won't help them. The seignorage will be deducted from their account whenever they finally decide to spend.
Why would you pay the central bank's exorbitant transaction fees when there are other miners willing to process your transactions for less? I suppose the bank could attempt to use its 51+% hash power to orphan blocks mined by miners not playing by the bank's rules, but couldn't the rest of us simply retaliate? If the bank doesn't play by OUR rules, their blocks aren't recognized. They might have a lot of hashing power, but so what? They opted not to use that power to gain 51% control of Bitcoin; they instead chose 100% control of Fedcoin (which seems like a strange choice). If that scenario occurs, I think I'll stick with Bitcoin.