The Boot Camp "Repair" option in the Programs and Features Control Panel did not restore any of the functionality. I managed to get things working properly again after a full re-install of the Boot Camp drivers. Someone has developed a utility to download the drivers directly in Windows, or you can follow these steps.
- Boot into Mac OS and run Boot Camp Assistant.
- Click the Action menu and select Download Windows Support Software. Make note of where the files are being downloaded.
- Copy the files to a USB drive, or upload them to cloud storage somewhere (recommended for future use).
- Reboot into Windows, copy the files from the USB or cloud, and run the setup.exe file. This will present a "Repair" option again, similar to the Control Panel option, but this really does reinstall all the drivers.
- Reboot into Windows again.
I found the keyboard and trackpad began working immediately after the Repair/Install completed, but rebooted as recommended. After rebooting, the keyboard and trackpad again didn't seem to be working, but they started working normally less than a minute after the Windows login screen appeared.