Comment 3 for bug 837032

Here's where I got with it back in March.

In this version it has a short flat cable connecting a uSD header with a larger board that intercepts the serial console cable and switches power to the board.

After some more discussion IIRC it became clear we could dispense with using the handshake from the serial console if we used duration of power interruption as the signal whether to come up in the recovery / reference SD or not. So if you turn off power for >10s, say, you guarantee the next boot comes up in recovery SD. For less than 5s say, you guarantee to boot in the test SD.

It was also discussed changing the power arrangements to use SD card socket power, which is much more convenient. At the moment it includes a MOSFET to switch the card's main power but that wouldn't be needed then. The downside is that a malicious test image could switch the SD Card power rail to select the reference / recovery SD.

I think these things are quite possible to build, especially as I am now in Taipei.