The Federal Reserve has been conducting Open Market Operations for many decades; the law was passed in 1913.
Open market operations (OMOs)--the purchase and sale of securities in
the open market by a central bank--are a key tool used by the Federal
Reserve in the implementation of monetary policy. Historically, the
Federal Reserve has used OMOs to adjust the supply of reserve balances
so as to keep the federal funds rate around the target federal funds
rate established by the Federal Open Market Committee (FOMC). OMOs are
conducted by the Trading Desk at the Federal Reserve Bank of New York.
The range of securities that the Federal Reserve is authorized to
purchase and sell is relatively limited. The authority to conduct OMOs
is found in section 14 of the Federal Reserve Act.
The law is defined here.
According to this article Open Market Operations were common in the 1920s. It's always been the Federal Reserve's preferred way of injecting liquidity into the bond market.
By the mid-1920s, the System became more active. Under the leadership
of Benjamin Strong, it initiated action to induce banks to borrow or
repay lending. From this modest start, open market operations became
the Federal Reserve’s principal and usually only means of changing
interest rates and bank reserves.
What's unique about the Federal Reserve's recent Quantitative Easing is the size of the interventions.
If the Treasury isn't able to make coupon payments, or pay for maturing bond/notes/bills they are in default. You don't have to ask for payments. Virtually all U.S.-based securities bonds/stocks/etc. exist as book-keeping entries in the Federal Reserve System.
For Treasuries there are various levels of Commercial Book-Entry Systems
At the top tier of CBES is the National Book-Entry System (NBES),
which is operated by the Federal Reserve Banks. For Treasury
securities, the Federal Reserve operates NBES in their capacity as the
fiscal agent of the U.S. Treasury. The Federal Reserve Banks maintain
book-entry accounts for depository institutions, the U.S. Treasury,
foreign central banks, and most government sponsored enterprises