Although it makes things more difficult, you don't really need the P2P protocol. You can send block and transaction data over *any* medium. Some suggestions I've heard are to post them (at least the block headers + links) on forums, in pastebins, or host http servers that serve the data yourself. Other ways of transferring would be though faux-video streams in video chat services. Or given absence of the internet, even broadcasting the block chain through radio. Possibly using a satellite (see http://www.coindesk.com/jeff-garzik-announces-partnership-launch-bitcoin-satellites-space/). Or indeed, mesh networks.

Of course a client would have to be written that can collect the data this way, but it is no technical barrier.

I wouldn't say it is impossible to kill politically, i mean given global North-Korea levels of tyranny, like taking away all technology from the common people, could of course foil bitcoin pretty much. But if that happens we'll have other things to worry about than bitcoin.