After learning how to add an unstable overlay to nixpkgs, being able to override individual service modules from unstable was something that I still struggled with until fairly recently. Hopefully this helps someone else looking to do common-but-not-very-obvious operation.
You can import unstable as a package override
I’ve had it in the past where I can specify in the package name (like pkgs.unstable.chromium). You’ll be able to do the same with service options as most allow you to set the package they use with a config option