Karabiner Elements has this template where caps lock can behave as an esc key if you tap it, and a ctrl key if you hold it down.

Thanks! That sounds exactly like what I need, and I'm already using Karabiner.

You can also use it for both ctrl and esc with a little help by Hammerspoon (http://hammerspoon.org), e.g. I've been using this for the past year and it works great https://github.com/jasonrudolph/keyboard#a-more-useful-caps-...