For those interested in what happened next: I managed to build and run sqlite, rsync, and dropbear SSH on the SD card using buildroot: http://buildroot.uclibc.org/
I quickly moved onto other interests as I couldn't work out any practical applications at the time.
Feel free to reach out to <me AT fernjager DOT net> if you have any ideas, or want to pick my brain on this project.
I also like the idea of having a tiny, relatively cheap, long running deaddrop wifi hotspot on batteries. Any idea of how long you could run it for from a button battery?
Could you create a half decent mesh network / wifi repeaters with these taped at various locations?
I could imagine a wifi hacking linux distribution like xiaopan might be quite fun too, or something with tunnelling / vpn that can connect to a hotspot and make it safe.
If there were some way to add a few sensors to it without making it much larger / power hungry, that could be really cool, especially considering how much space you have for logging.
I'd like to see if one could be surreptitiously built into a usb cable, or wall wart, so it's powered when the usb cable is plugged in, but otherwise there's nothing obviously special about the usb cable.
Potential use case: turn a simple printer equipped with an sd-card slot (to print photos directly on the printer) into a manual wifi printer. The job would be converted to, say, jpeg, sent on the sd-card and then you would have to print it manually.
Other use case: be able to send any file to this SD card, this way you could easily transfer pictures, videos and music to a non WiFi multimedia device, like a TV Tuner, MP3 Player, etc...
The SD card has a script that monitors filesystem changes, and would push the images via curl to the client connected to it through WiFi.