If there would be any "forking" to be done - or modification to the main branch, I think it would be the inclusion of such a channel.
Interesting idea. But, I really think most scenarios where it might be really useful might preclude its use.
But with some additional effort we might arrive at something disaster resilient.
*generator, solar, or manual crank powered laptop.
*bitmessage client with pre-defined emergency channels
*bitmessage spitting a stream out to a radio transmitter that broadcasts the blockchain via morse code.
*radio reception for same
Imagine such a setup - and someone travelling up or down the East Coast of the US (just as an example). As they passed through different reception zones, they would merge blockchains, and act as a crypto-pony-express.
If nothing else - I think I have a budding scenario for a sci-fi short story.
=kewal=
since you are just duplicating the same data at every node and broadcasting it to every other node you are connected too, it would naturally propagate across all networks so long as there was the occasional tunnel linking networks to their neighbors even if your node isnt aware of any networks outside of your immediate surroundings.