I got a nice overview image in the Windows app, where you can click on the display that the hardware is attached to. I also got global hotkeys working, so you can actually twist the device and the screen changes! Pretty cool...

Next issue: the digispark board has one of those PCB-track USB plugs, and it's a little bit short, so in my actual monitor I think only the power lines are making contact (they're a bit longer). I also seem to be having problems with some hubs - even with a USB extension, it's hit or miss whether the 'keyboard' is detected, whereas it's fine plugged into a standalone powered hub. It's not clear if this is an issue with borderline spec USB from V-USB or another power/connector issue. The real answer is to make my own dongle, I guess.