It would be counterproductive for Jaan to sell their USTs.
Selling USTs in exchange for USD would result in higher UST rates. More global capital would then flow into USTs, which would further increase demand for USD, which would mean further USD appreciation.
In a nutshell, selling USTs would have the opposite effect. as interest rate differentials widen between countries (selling bonds increases their rates), USD would strengthen even further against JPY (and other currencies).
What would help is if the BOJ borrowed USD from the Fed using their U.S. Treasury holdings as collateral. Once in possession of the USD, they then sell them in the market for JPY.
Once USD has weakened enough, they can buy it back and pay back the loan.
Theoretically, this should be possible with the Fed’s FIMA Repo Facility.
Of course, this would backfire if the USD continued to strengthen across the board (which it has been).
The best solution would be a globally coordinated move by all major central banks to weaken the USD similar to the Plaza Accord, or the more recent (and secretive) Shanghai Accord.