It's actually quite simple and readily available with PAM duress [0] (at least on Linux, I'm not sure about PAM on Mac). It was also discussed here already [1]. Still, you should consider that doing so might not work in your favor, especially if the fact that you used a duress code becomes obvious.

[0] https://github.com/nuvious/pam-duress

[1] https://news.ycombinator.com/item?id=28267975

Yeah it was more theoretical speculation, probably not doable on Mac (and I don't see Apple adding this feature tbh). I wouldn't be surprised if someone's implemented something like that in Linux though.