Qutebrowser runs on top of Blink, which is the same guts that are in Chrome. So if Widevine is packaged for some other browser that is available on your distro, you can pass in the QT arg

"widevine-path=/path/to/google/chrome/WidevineCdm/_platform_specific/linux_x64/libwidevinecdm.so"

and it should work. This even works on NixOS, although you have to pull the relevant path out of the Nix store, and I don't know a good way to do that automatically that can fit inside a qutebrowser config.