Let's all be Nozickians for a second. Let's forget this ethical baggage that these questions get saddled with. I think it's poorly reasoned ethics, but let's set that aside for a second.
Let's say that the only corporate entities we have are contractually entered into, and let's call the contracts "states" if they are meant to facilitate collective action to solve social problems by (voluntarily) empowering a democratically governed authority to raise taxes and spend on the general welfare.
So we're all Nozickians and let's say we're not even Keynesians. What would our voluntary, private governments do in the face of these negative interest rates on government bonds? Forget Keynesian models about exactly why one might want to issue more government debt. Simply as a matter of private reaction to relative prices.
Would a Nozickian, non-Keynesian, private government survive if it insisted on restricting its own access to credit markets precisely at the time when issuing bonds was inexpensive - indeed profitable? Even aside from Keynesian macroeconomic concerns presumably a Nozickian private government would still have things that its voluntary citizens would want to get done, right? And presumably when the price of doing things got cheaper you would see an income effect - a marginal increase in the demand to do more things, right? Every rational Nozickian government - even without Keynesian predilections - would be doing expansionary fiscal policy right now.
There's only one reason why a rational private entity facing these sorts of interest rates on their bonds would not be expanding right now and that's if they were a profit seeking entity that saw reduced profitability in the future. But governments - whether Nozickian and private or public - are not profit-seeking entities. Even insofar as Nozick or Lysander Spooner or others conceive of voluntary private government, it's still not a profit seeking government, so it still ought to embrace fiscal policy under circumstances similar to what we're experiencing now (albeit probably not to the extent that a Nozickian state with Keynesian predilections would).