I'd expect the Pi Zero W to be fast enough to pass through the MJPEG stream and impersonate the keyboard, which should cut the cost by $20+ plus however much you save on not needing a fancy case.
>>Scaevo+(OP)
Yeah that's true. I stopped thinking about the Pi Zero W early in the project because I assumed it would choke on video encoding. When I discovered that the HDMI dongle was doing the heavy lifting, I forgot to revisit that assumption. I'm going to order an USB to micro-USB adaptor to test it out because it would slim things down significantly.
>>mtlync+7a
Woops, forgot the major issue-- the Pi Zero only has one USB port (the other is just for power), so you couldn't get video frames in at the same time as you pretended to be a keyboard.