Alternative question: Anybody successfully running a Neo2 keyboard layout on macOS Sierra yet?

I might be wrong but I think @HMeine is

No, Sierra killed Karabiner, so layers 4-6 are not functional. 😔

I couldn't even get the custom keyboard to show up

I did not install from scratch, but upgraded to Sierra. Still works as intended. neo.layout + .icns under /Library/Keyboard Layouts

And modes 1-3 work as intended?

yes.

Thank you, will give it another try.

Just found github.com/jgosmann/Karabine… – layer 4+ now work for me, at least with the internal keyboard.