Working healthcare, I tell all other healthcare / first responders “I hope you have a boring shift” (every once in a while a younger person objects, but they are almost immediately hushed / corrected by their senior coworker).
But it occurs to me that many office / sedentary workers might not find this to be a blessing. What would I say to a desk worker that might be equivalent?
Whistling comes from when whistling was used to communicate to stagehands to lower and lift the various backdops that used sand bags as counterweights. Whistling could cause someone up in the grid to send something down that was heavy, injuring or killing people.
The thing about the Scottish play that you mentioned (which is what most theatre folks call it) is more of an actual superstition, but may well stem from the fact that the play has a lot in it that can cause injury if not done safely, so the thought is that a lot of productions of the play can feel cursed, so that turned into referencing the play feeling cursed.