It's not that it's not interesting, as it is a great idea. Stuff that I consider when feature requests are made are things like what the mass appeal would be like. Yeah, this would be a great thing to have, but it would require some type of UI element. If you pair that up with how many people might actually use something like this, it's kind of overloading the UI unnecessarily and penalizing almost everybody for the benefit of very few. Since, 'trigger' items are kind of out front (you know... voice command, mouse button, keyboard, joystick, unrecognized command, etc.), you'd have to also add, 'When Window Loses Focus' out front too (and not get to stuff it down into a submenu like a normal action). It's kind of why I haven't added variable change triggers (yet) - the UI would have to be overhauled to be able to manage x number of triggers and still maintain some balance that doesn't scare folks off (it's already way too overloaded, no?).
Again, it's not a bad idea - it's just something that's very specific that, as you said, *is* achievable through actions that the user can create (or via plugin or inline function).