I am trying to set up a custom shortcut to map Esc to Esc:Esc, i.e. simulate a double press on Esc key, and so far have had no luck.
(systemsettings -> gestures and shortcuts -> custom shortcuts)

I also tried Escape:Escape, didn't work either.

It would be better to have a comprehensive list of how every key should be represented.

Elaborate more on the scenario.
Sending shortcuts to inactive windows basically doesn't work at all (client toolkits simply ignore sythetic events)
Esc:Esc should work, but because the Esc key is also pressed as trigger, the press/release order will likely be messed up (you can try that on "xev", run it from konsole)

Thanks. I changed the trigger to shift + esc, and now it's working.