The Bitcoin blockchain can be kept up-to-date with less than a 2400 baud connection. ... Or is this concept simply wildly-unrealistic, pie-in-the-sky speculation?
Maintaining the blockchain requires bi-directional communication. Your proposal is basically transmit only. that works ok for something like price quotes, where if you miss one, you just wait for the next time it is quoted. If you miss a part of the block chain, you need to request the data to be retransmitted. What if your town looses power for 8 hours? You need all that data to be rebroadcast when power comes back up.
I could see the broadcast for the major part of the data, and your own internet connection to request missed blocks.