The NAP certainly does permit the use of force by agents of the victims of aggression. If A witnesses the commission of a crime against B by C, and B yells for help, why should he not be permitted to delegate his right of self-defense to A? Or if A stops the crime, without B's asking for help, B's right of self-defense has been nonetheless been exercised through the agency of A. The difference is that the delegation was implicit, rather than explicit.
The NAP makes assumptions of prevailing behavior. There is a general assumption that property owners do not want their front yards used as latrines, even if there is no sign making that prohibition explicit. There is a general assumption that a closed door means you aren't invited to enter. There is a general assumption that if a person is under attack, that person wants to be saved. Exceptions not made explicit by the exceptors would not be enforced, because people operate under assumptions derived from prevailing behavior.
Also, since most law would be developed as codes of conduct by DROs and insurers, that sort of thing would be worked out ahead of time. So whereas B from my example has the right to retroactively refuse A's interference in the crime, it is unlikely that any DRO would act upon such a complaint.