>> As stated it's wrong
You misunderstood that comment. He was saying that my claim (that using an upside-down rook is illegal in serious play) is wrong as stated, because it's only true for FIDE rules, not USCF rules. He wasn't saying the act of using a rook that way is wrong (morally or otherwise).
> if you want to claim the rules forbid this act, you should be able to cite a rule
No, this isn't how rules work. The rules of a board game describe all the ways you're allowed to move the pieces, under what circumstances pieces can become other pieces, and so on. If some piece transformation isn't discussed in the rules, then it's not allowed. Otherwise, I could on a random turn say "I declare all my pawns to now be queens". There's no specific rule that says you can't do that, but nevertheless, the rules forbid it implicitly by not mentioning it. Similarly, the fact that they never mention that a rook can become a queen means that in fact, it can't.
So in fact the rule you're looking for is on page 7 of the PDF you linked:
> If a player having the move [...] promotes a pawn, the choice of the piece is finalised, when the piece has touched the square of promotion.
"The piece". Not a different piece having been turned upside down.
Let's take an analogy to real-world laws. Imagine you live in a country that says apples are taxed at 1 cent per apple. Now, imagine a shop turns all the apples upside-down, and declares that they consider upside-down apples to actually be bananas, so they don't have to pay the tax. Is this legal? No! Even though the law doesn't mention anything about whether you can or can't do that, nor does it give a mathematically precise definition of what an apple or banana is. Even though it's not explicitly forbidden, it is still not allowed.
> I'm also, across both of these descendant threads, annoyed with responses not readily engaging with my legitimate inquiry about why forbidding the use of the inverted rook as a promoted queen proxy may be the case. It's a neat quirk or curiosity to me
In another subthread I gave five plausible reasons why it makes sense not to allow it. Link: https://news.ycombinator.com/item?id=45512086