I really don't think we need a rule for early verbal announcements. However, if you feel a need to enforce such a rule, I like my earlier suggestion using the first card pitched as opposed to the first riffle. Holding a player to his premature announcements while the dealer is shuffling seems a bit too soon. I don't believe a "rule" is necessary. Wait for the proper time to act and there's nothing to worry about.
There were similar discussions years ago, but they pertained more towards out of turn action. I suggested that "deliberate" action out of turn should be handled different from accidental. The argument then was; how do we know when action was intentional or deliberate? This is what I was trying to address...the described premature action is positively deliberate, no question about it. I suggested that the deliberate action must stand. Deliberate premature verbal bets will be treated the same as any other bet in turn, it can not be retracted and there will be no recourse if the hand is fouled. A couple of bad experiences and the "early" bettor should learn his lesson.