I believe it is against the rules...
My understanding is this covers it twice.
Rule 12 - Player Conduct,
Section 3 - Unsportsmanlike Conduct
Article 1 - Prohibited Acts
(h) - -
"Removal of his helmet by a player in the field of play, or the end zone, during
a celebration, OR DURING A CONFRONTATION WITH A GAME OFFICIAL or any other
player."
and
Rule 13 - Non-Player Conduct
Section 1 - Non-Player Conduct / Attendants on Field Only on Team Timeouts
"During any team timeout, all playing rules continue in force. Representatives of either team are prohibited from entering the field unless they are incoming substitutes, or team attendants or trainers entering to provide for for the welfare of a player, (MY all Caps) AND ANY GAME-TYPE ACTIVITIES ARE PROHIBITED ON THE FIELD OF PLAY."