I have had two ideas around this theme, this being the easiest to implement. The other may still turn up (although I keep looking at it, and hurriedly thinking of something else), but not for some time, and not as an EV.
SPOILER ALERT has twelve letters, of course, so is a shoo-in for a regular sized grid. The idea of a spoiler alert is that shortly after its appearance there is going to be a factoid or datum that tells you something you might rather not know until you have seen a film or read a book or so on. What would that be in a crossword context?
Obviously it would be something that helps you with solving the puzzle, but which you might prefer to disregard, at least for a while. At that point I thought of the classic “kickself” poser where you are asked to rearrange the letters of NEW DOOR to make one word. It seemed to me that I could use my alert to say there was one answer on the unclued bottom row, and that would be literally true. Of course, ONE ANSWER only has nine letters against SPOILER ALERT’s twelve – so both entries would have three unchecked letters.
The process of augmenting clued answers with additional letters to reveal ONE ANSWER came to mind next, with the associated idea of hiding the necessary definitions as redundant elements in the clues. I’m not quite sure where READ THIS came from; in retrospect, it doesn’t seem such a good idea, without being exactly wrong.
A final qualm was whether to have a bar in the unclued rows: SPOILER ALERT isn’t in Chambers, and a bar might help people less familiar with it. (I still wonder whether there were solvers unacquainted with web etiquette – or lack of, I suppose – to wonder what a spoiler alert was.) In the end, the difficulties a bar might present for parsing ONE ANSWER decided me against it, but it was close enough that I absent-mindedly submitted a version with the blank grid in one form and the filled grid in the other.