Don't know if anyone's ever gotten it before me, but I did manage to get the secret ending and solve the mystery of the obelisk's inscription. (Mercifcully, unlike the text adventure games this is styled after, once the ending is triggered, the puzzle is solved automatically instead of spending hours figuring out what to do.)
One important thing I'd add to the developer's hints: the idiom is the same as the underlined one used by the Oracle, but sand is replaced by "time".
Also, if you're not sure you've successfully clicked on the redundant element at the right time, click on it before getting to the idiom's second appearance and remember what happens when you do click it. That will give you the experiential reference you need to know.
OK, if you're down here and are the dev, some questions about the ending:
* Is the ultimate solution to the obelisk's elevator functionality to have three people who have recently been...transferred?...to stand at the positions indicated by the inscription with the bowls, and one still-living person to do the same? I know it doesn't matter since, regardless of the solution, the game auto-solves it for you, but as someone who's played a lot of adventure games, graphical, text, and hybrid, I'm still curious. :D
* When Olivia arrives at the artists' chamber, does she lose her face at that point, given she's doomed to a different sort of eternal life than in the bad ending?
*Aside from explaining how the paintings get made, what is the office-like artists' chamber meant to represent? I must admit the symbolism there, if any, is lost on me.
Anyways, I didn't like the ending at first, until I realized what the Oracle meant when she was talking to me, the player: I can't cheat Olivia's ultimate fate - she will enter into the afterlife one way or another (not exactly sure if being transferred(?) or managing to enter the artists' chamber counts as dying). But I can make that fate bittersweet.
But yeah, without the hints you provided previously, I'd have no idea how to achieve the secret ending. Even if I'd thought to go back and enter Olivia's actual dying words to the Oracle, the idiom sounds like "when the sandy blood of Tyler flows (slips) through your hands, exit the game using the in-game exit button", not "when this idiom is repeated similarly in the game, exit the game using the in-game exit button"...though that's my adventure game brain working there. While I agree the solution is definitely overly-obtuse, I do think a simple change would make it much easier to achieve. Specifically, if you've gotten the Oracle's advice using Olivia's true last words, that locks you in to the secret ending. As such, once the idiom is stated in dialogue, Oliva suddenly realizes the Oracle's meaning and, thus, knows it's time to LET GO (possibly allowing the player to actually type that in).