Well… I got as far as seeing ‘Press any key to boot from CD or DVD…’ from the Windows 10 2004 ISO, but it doesn’t respond to keypresses. Tried passing through the USB keyboard and mouse individually as well as passing through the entire xHCI USB controller. I have a second USB controller on the way. It’s the one recommended here.
edit Got the Windows installation running. I’m guessing that adding VirtIO Keyboard is what did the trick. I had just proceeded to the Looking Glass instructions since I knew I was going to want it eventually.