To the best of my knowledge, there is NO rule specifically denying anyone the option to tell the truth about one's holding WHEN HEAD'S UP.
Robert's Rules of PokerShow one, show all.
Players are entitled to receive equal access to information about the contents of another player’s hand. After a deal, if cards are shown to another player, every player at the table has a right to see those cards. During a deal, cards that were shown to an active player who might have a further wagering decision on that betting round must immediately be shown to all the other players. If the player who saw the cards is not involved in the deal, or cannot use the information in wagering, the information should be withheld until the betting is over, so it does not affect the normal outcome of the deal. Cards shown to a person who has no more wagering decisions on that betting round, but might use the information on a later betting round, should be shown to the other players at the conclusion of that betting round. If only a portion of the hand has been shown, there is no requirement to show any of the unseen cards. The shown cards are treated as given in the preceding part of this rule.
The no disclosure issue is probably due to the
TDA RulesPlayers are obligated to protect the other players in the tournament at all times. Therefore, players, whether in the hand or not, may not:
- Disclose contents of live or folded hands
- Read a hand that hasn’t been tabled
- Advise or criticize play before the action is complete
The one-player-to-a-hand rule will be enforced.
However, if you are HEADS UP then there are no other players to protect as your opponent doesn't count.