Assuming the total co-operation of the Republican dominated US Congress, these bonds could be permanently confiscated in order to fund the wall.
Such an act would probably be without precedent.
Iran, for example, was unable to access their US bond holdings until the recent treaty allowed them to. In this case, legislation didn't permanently seize the funds - just kept the owners from accessing it while the dispute between the two nations continued.
Of course, seizing the international reserves of a country in order to make them pay for something is hardly in the best interests of anyone. Yet here we are and Donald Trump is about to be sworn in as president.
We can look forward to some very interesting times ahead. But not "good" interesting.