[kiwiref]

Coming from a different background in terms of rules, it was interesting to follow this discussion, particularly after the actual rule was posted.
IMO, the rule implies that the request has to be made at the appropriate time because there is no requirement for the officials to keep track of and administer pre-requests. In essence, the rule is "coach asks, referee grants if appropriate", not "coach asks, referee grants when the opportunity comes".
Giving heads up on an upcoming request is a different case altogether, obviously.
Cheers.