A stand-in director at the club said he had been called to deal with this situation at an evening's duplicate, and asked my advice.
West asked for an explanation of the 3NT opener and was told: "A running 7-card minor with no outside ace". This is exactly what it says on NS's convention card. NS are a regular partnership, but never venture outside the club. EW are an experienced pair who often play in tournaments. I don't think they had any agreed system to defending 3NT openers.
Result: 5♠X(E)-2, NS +300.
This was a good result for NS, as most pairs finished in 4♠-1.
EW wanted the hand recorded as a psyche. North argued that it was a deviation rather than a psyche. I didn't learn what South said about his own opening bid, I suggested to the TD it would have been a good idea to ask him, but if he just assumed that this hand was likely to end up in 3NT so he might as well bid it now and not reveal anything about the hand, what should the TD do? Is recording the hand enough?