The central bank already owns about a third of all outstanding government bonds and every month buys about as much as the government issues. Not much room to do more there.
Interest rates could be taken lower, but the costs now seem a lot easier to spot than the benefits.
As for intervention to sell the yen and drive it lower, that historically has been a game that only works longer-term when all sides are playing toward the same end.
Certainly, Japan can do yet more fiscal stimulus, and may well be likely to, with the alternative being admitting that the Abenomics experiment is a busted flush.
The weakening dollar since the Group of 20 meeting in January has fueled speculation of a tacit agreement, or secret deal, which might make yen strength simply collateral damage to solving bigger issues. Such a deal is highly unlikely, though it is certainly true that in taking the pressure off the Chinese yuan, global policymakers find themselves with one huge and disruptive issue less with which to contend.
More likely is that major players truly are happy to behave as if they've reached a deal to limit the dollar's rise. That takes us back to the effective limits of monetary policy and beggar-thy-neighbor currency depreciations.
"The winners of the currency war battles may have decided that they were not benefiting enough to offset the negative impact of the ancillary asset market volatility that emerged," Steven Englander, currency strategist at Citibank, wrote to clients.
"Basically, they were acknowledging policy ineffectiveness or at least monetary policy ineffectiveness, and the G20 statement pretty much admitted that."
The upshot is more volatility, as governments and central banks will be reluctant to intervene or push rates lower, leaving markets to move as they will.
The bigger question is how investors and markets react once they realize they are working without the safety net they've enjoyed since the financial crisis.