The Digisparks and tilt switches have arrived. I also got a working C# app to test the rotation part. Current issues:

• The digispark driver won't load on Windows 10
• The Windows EnumDisplayDevices() API call produces "interesting" output - it would be nice if the IDs matched up with those in the Displays control panel. Otherwise, I'd want an OSD 'identify' option like that control panel does - so how to figure out which displays match up to which areas of the desktop?
• It's conceivable you'd want more than one of these things, so I'm thinking about adding a 1-wire chip, just to get the unique ID. DS18B20 temperature sensors are about $2 a pop (the simple 'serial number' chips aren't really much cheaper, and I already have some temperature sensors).