> we need a way for engineers and scientists to feel accountable for the outcomes of their work, and to straight out refuse working on such projects.

One difficulty, not just here but with pacifism in general, is the asymmetry of violence. For your scheme to work, we would need close to 100% cooperation from engineers. For war hawk politicians to get their weapons, they need just a handful of engineers.

"Today we were unlucky, but remember we only have to be lucky once. You will have to be lucky always." — https://en.wikipedia.org/wiki/Brighton_hotel_bombing