I think that the U.S. government would enact a wealth tax rather than default on its debt.
Other countries that have defaulted have not had the option of enacting wealth taxes. When you are in a banana republic with shaky government finances and you have a lot of wealth, you send that wealth over to the United States, where your government cannot get to it. That "safe haven" motive is what keeps the dollar so strong. Anyway, by the time the banana republic gets around to enacting a wealth tax, all the wealth has fled the country and there is nothing left to tax. So the banana republic defaults.
As the U.S. government's finances deteriorate, it will strengthen its hold on its citizens' wealth. My guess is that you will see tighter laws that restrict your ability to hide wealth overseas and much more enforcement of those laws.
Basically, if a banana republic says to us, "Help us keep the wealth of our citizens," we can say no. On the other hand, if we tell another country, "Help us keep the wealth of our citizens," that country will co-operate. This asymmetry reflects the distribution of military power.
So what I am saying is that the ultimate guarantor against a U.S. government default is the U.S. Navy. Because of the navy, the U.S. government can control the policies of other governments. Because it can control the policies of other governments, the U.S. government is in a position to dictate whose wealth can flee where. Because the U.S. government can stop our wealth from fleeing, the U.S. could enact a wealth tax. Because it could enact a wealth tax, the U.S. is unlikely to default on its debt.