I have a pipe dream of setting up self-sufficient devices like that, and stowing them on public transport (e.g. buses, trains), and letting them travel around opportunistically connecting to open wifi hotspots.

If you have enough of them, and if you have clever enough software, you can be reasonably confident that at least one "server" will be up at any given time.

Sounds a lot like this bit from Kill Process by William Hertling:

"The little computers run a secure variant of Linux, with a single open port, protected with heavy encryption. Part of the computer board contains a sensitive accelerometer, which means I can detect when the computer is moved.

When I travel, I find coffee shops and homes with wi-fi signals and flat roofs, and I toss one of these onto the roof.

If you were to find one, pick it up, and look at it, you might not be sure what it was. If you plug a headphone into the jack, it plays pirate music stations.

Of course, that’s what it does only if it’s been moved or if the battery level drops too low. Because when the accelerometer detects motion, the code I wrote replaces my extensive software with a simple dummy music app and erases the remaining storage a hundred times over.

If it hasn’t been moved, and the battery level has never dropped too low, then it does what it’s supposed to do: operate as part of my private onion routing network with hundreds of nodes to disguise my digital trail so others can’t trace my location.”