RasPi Zero W Keyboard emulation works fine except:
At boot, older PCs hang during POST if the keyboard is attached via KVM.
All other scenarios (booting without Pi connected, booting with Pi connected directly to PC not KVM, using kvm.py to switch KVM ports) work fine.
Attempts to read reports from keyboard during boot yield no data. PC doesn't seem to be waiting for keyboard.
I found this post. Using that report descriptor cleared up all the PC issues. I still can't get the system to POST when the Pi is connected via the KVM.