Mozilla are lucky enough to be able to afford a code-signing certificate and I'm pretty sure their build process can be adjusted to sign binaries by default.

It's unfortunate that a LetsEncrypt style project can't be done for code signing, due to malware/admin overhead.

There's no need to do that. Just right-click on Servo.app and select "Open". A dialogue will pop up asking you if you're absolutely certain and off you go.