Sundown is a moment in time. Grains falling through an hourglass is a measurement of time. The only way I could see "sundown" and "a certain number of grains falling through an hourglass" being Optionlocks is if those were somehow unhinged from the passage of time--but then they wouldn't really be "sundown" and "an hourglass" as we know it. For example, the story of Amaterasu and the Cave is an Optionlock, and the endgoal of the story is sunrise, but that's because the Sun Goddess Amaterasu is throwing a hissy fit, and (solar) time has stopped because of it. Similarly--and to jump examples, my apologies, but I can't do it with the hourglass--if two characters were fighting in a stopped clock, and another character had the power to make the clock jump forward a notch at whim, then yes, that would be an Optionlock. I'm imagining a character tied to one of the gearteeth, and each time the bad guy is... I don't know, reminded of his lost love, he pushes the gears forward another tooth, towards the crunchy intersection. But again, this is a clock unmoored from time, less a "clock" than a steampunk gearbox in the shape of a clock.
The actual time, the numbers, don't really matter to me. That's what I meant by them not being "sacrosanct." What matters to me is that hinging with time, that constant progression towards the climax. If I need to finish a task before sundown, and I can see the sun right in front of me, the actual numbers don't matter, because I can make up my own versions in my head. The sun is... "that" high. And now it's, like, "half that" high. Is that 5:00 to 5:30, or 3:00 to 4:30? It doesn't really matter. What matters is I had "that" much time left, and now half of it is gone. (Like, who cares about the distance the sun travels in the sky? What matters is the time it represents having passed.)
This is also the difference between a story where the characters visit X cities, and story where the characters travel Y miles, passing through X cities. The first is an Optionlock, because the time doesn't matter. They stay in city 1, twiddle fingers, then eventually leave. Once they hit all X cities, the story ends. But a story about a pilgrimage, where they have to travel Y miles and the cities are just rest stops, has a different feel. In the second story, distance (which is an analogue for time, here) is constantly closing. As they travel along the road, they keep getting nearer and nearer to their destination. Even if they stop, it doesn't matter, because they know they can't stay.
The difference is between this:
EDIT: Or to use my previous explanation, in the first example, "30 miles away from the holy site" and "29.5 miles from the holy site" don't make a difference, because we're measuring the story in cities, and individual miles don't matter. But in the second example, "29.5 miles from the holy site" is exactly 0.5 miles more somber than "30 miles away from the holy site," because we're 0.5 miles closer.
EDIT: And to take your airplane example, the moment he sets foot again, he's back under the yoke of the flow of time, so it's still a Timelock. At least how I see it.