The ACBL has elected (Item 7 under "Elections" in the back of TFLB) that "Law 40B3: A partnership, by prior agreement, may not vary its understanding during the auction or play following a question asked, a response to a question or any irregularity."
It occurs to me, having had more time to think about it, there was was no implication in Hardy's exposition, nor, I think, in my OP summary of that, of any agreement, prior or otherwise, as to the meaning of 1
♥ in the given circumstance. Dealer's LHO has a 4 level preempt in hearts. This is AI to the dealing side and UI to the dealer's RHO (Law 16D). A one heart opening in this circumstance is effectively a (perfectly legal, IMO) psych.
What's happened is that Max (via his friend who presented him with the problem) was faced with an unusual circumstance and asked "what would you do?" There is no implication even of partnership understanding here - what is the frequency of a 4
♥ opening out of turn on your left when you hold this hand? The phrase "once in a blue moon" comes to mind. No, Max is suggesting that he would hope that partner would recognize 1
♥ as a kind of "advanced cue bid" and recognize that Max is trying to show the hand he actually has.
I can't imagine that anyone in his right mind would bid 1
♥ naturally in this circumstance. Therefore, anyone in his right mind, partner or opponent, will recognize that 1
♥ is not natural. So what is it? The information needed to figure that out is available to both opponents and partner. The difference, btw, in that availability is that "Top and Bottom Cue Bids" will be marked on the system card, and the opponents can read that, while partner has to remember it.
"You won't be able to make the bid a second time."
I disagree. It takes more than two incidents "once in a blue moon" to make an implicit understanding. Even if this unusual occurrence happens twice in a week (or a session) the occurrence itself is unusual enough that I would not expect the bid to fall afoul of 'implicit understanding'. Not in any rational jurisdiction.
Added thought: it seems to me that the claim that one instance establishes a partnership understanding implies that both players have perfect memories. I can't speak for anyone else, but
my memory certainly isn't perfect!