The answer is already in the submission.

  They would gladly facilitate an option the Treasury pursues in its role as fiscal agent, since they can always claim that it was the Treasury’s decision and they had no other option as fiscal agent (which happens to be true).

The end result is the Federal Reserve would be forced to follow its unwritten 0th commandment: maintain the integrity of the payments system. As a consequence of that imperative it would be operationally forced to continue to ensure Treasury auctions and payments continued to go through. Presumably accounting gimmicks would be employed to avoid “technically” exceeding the debt ceiling, like simply allowing Treasury to overdraw. But at the end of the day there would be no constitutional crisis. At least not a forced one. Of course we shouldn’t underestimate the power of stupidity so it can’t be ruled out entirely.