Hmm. Interesting. did not know about 922(o) wavers. That explains why DC is so high on the list.
The machinegun waiver:
(2) [The prohibition of otherwise law-abiding US citizens owning machineguns made after 1986] does not apply with respect to—
(A) a transfer ... by, or possession by or under the authority of, the United States or any department or agency thereof or a State, or a department, agency, or political subdivision thereof
...at which point it just turns into a standard NFA Form 4 transfer. Such waivers are the norm for any gov’t agency possessing full-auto weapons, so the precedent is there.