True, there would probably be someone with a dial-up modem or satellite dish internet. Rarer would be someone who has both that and the wired internet that has the outage, but if it's a big enough segment to matter, out of a million people there's bound to be a multi-home geek.
ISP network cuts are just your local area. If you still have communication with the rest of your area, it would probably be something like 1/1000 of the world or less. Block generation in the segment would take several hours per block.
I favour the plan to monitor if the frequency of blocks received drops too slow. That covers a large range of possibilities.
But there will be no irc server to bootstrap from.Which doesn't matter because you can't access sourceforge to download the software either.
If you've ever been connected before, you don't need IRC to bootstrap anymore. Even if you haven't, you can bootstrap from seed nodes. IRC is completely redundant since 0.3.0.