I can definitely see the appeal, and I think it'd be a welcome big picture move for the "slow-burn" type of zombie apocalypse we're used to. But under these circumstances, I'm just not sure the UN can assemble the Security Council, or even a quorum, much less request member states to divert much-needed resources from their horrific domestic problems to help out the US (most member states would probably assume they can take care of it themselves, anyways). Maybe if they're in contact with the Secretary General as well as the Prime Minister, it makes more sense. 

I'm not bashing the idea of a Blue Helmet aid worker, I really do think it's fantastic and I'd love to see it incorporated elsewhere. But for this specific set of circumstances I'm just not sure. You're more than welcome to keep the UN aspect if you like, I just feel in light of the global situation a fact-finding mission might be more believable than an aid mission.  

Just some thoughts.