In other words, QE to infinity as required.
That will be the solution the Fed will accept when the Treasury prints bonds that have no buyers except for the Fed and its cartel of banks.
Of course they will stop before infinity is reached. The only question is, "What will stop them?" Especially if they engage in off balance sheet shenanigans like certain types of swaps and mutual devaluation with other central banks for example.
Will the economy turn around and a combination of growth and inflation overcome the excessive debt burdens? Or will they simply reach a point and engage in a de facto default as Russia did and reissue a 'new dollar?'
These are the important questions to think about. But I doubt very much that the Fed will stand aside and allow a hard default on the bonds. They will attempt to make the dollar look good by making all other currencies look bad.
The default will be de facto and as nominal as the cynical toleration of a denaro buffo will allow.