Posted 2012-February-05, 17:15
Not sure who you want to give a ruling, since it is a question in a contest from the ACBL Bulletin, to be tallied and discussed next month.
But let us assume your partner, who balanced with 2NT after not acting on the previous round, knows now ---as you do now --- that 2S was to play. I don't think the alert regulations address a new suit correction of lefty's preempt after you have opened the bidding. They seem to only address new suit bids by responder to opener's preempt in an uncontested auction.
Your only decision here is whether 2NT by a previously silent partner is natural (suggesting he was willing to defend 2H --even undoubled) or is an attempt to back in with something like 5 diamonds and 3 or 4 clubs. I am voting for a bid of 3♦ by opener when I submit mine, but we shall see.
"Bidding Spades to show spades can work well." (Kenberg)