Well, I guess that's the answer I should've expected. But imagine this:
The government begins some big spending program. Say, the Green New Deal, which I've heard might cost up to 90 trillion dollars, which obviously would mean a dramatic tax hike. But whatever, this is a hypothetical so you can substitute the Green New Deal for anything you want.
Or for whatever reason, let's say. Companies and affluent persons/parties begin pulling their money out of the United States at such an alarming and sudden rate that the national GDP is cut in half.
So let's say, for purposes of simplification, the GDP goes from 20 trillion to 10 trillion overnight. And there's no indication that our country would be getting that money back any time soon, if ever. At the same time, the increase in the national debt over time was largely predicated upon the assumption that such an event would not occur (that is, that we'd be rich enough to ensure that the debt was manageable, that if need be we could begin shrinking it significantly over a certain period of time, that we could afford to pay down the annual interest on said debt, etc).
So at that point the national debt is doubled for all intents and purposes, so far as our ability to pay goes.
So at that point, other countries would be the ones in a position to tax that wealth. Even if they aren't, the fact remains that we can't because they gave that money somewhere to go to. Obviously there are a lot of small tax haven countries that don't hold a lot of US debt so this wouldn't work to simply target them. Rather, we would simply have to consider "the rest of the world" as in contrast to the United States.
If the "rest of the world" is holding on to the wealth that we could be using to pay off or otherwise manage the national debt to whatever degree, and thereby preventing us from using that money for that purpose, I would propose that we deduct from the national debt according to a formula that calculates how much we would've (or, could've, under normal tax rates, accounting for the fact that realistically most of it would not go towards that purpose under normal conditions) collected in taxes. Under these circumstances I'd even be hesitant to call it sovereign default, but rather a fine or penalty imposed for damages incurred to our economy, of a severity that was equivalent to the severity of said damage.
Understand, of course, that the measure I've described would be reserved for a national emergency like what I've described above. This kind of deduction would NOT be made every year under normal circumstances to compensate for normal rates of capital flight, because of course to do it to any degree harms the reputation of the United States as a debtor, and quite severely. But if we're stuck with this unpayable debt because they bailed on us, then I'd hardly call the US government the most heinous offender here.