Here are case plays from the NFHS:
1.5.5 SITUATION A: During the umpires’ checking of equipment prior to the
game, it is observed that several bats are in warming devices. RULING: All bats
found in the warming devices are illegal and removed from the game.
1.5.5 SITUATION B: In the fourth inning after B3 hits a homerun, the plate
umpire notices while moving the bat from home plate that the barrel of the bat
feels extremely warm. The umpire then observes several other bats in the dugout
being stored in warming devices. RULING: B3 is ruled out and the head coach is
issued a warning. The next player not properly equipped will result in the player
and head coach being restricted to the dugout. All bats found in warming devices
are illegal and removed from the game. (3-6-1; 7-4-2)
These demonstrate that bat warming is illegal and not without penalties. However, I would have a hard time considering a player's body or part of her legal uniform as being "bat warming devices".
I really think that a "bat warming device" would be something beyond a player's own body temperature or clothing. Any heating from those sources would have to be so negligible that they have no measurable effect on the bat's performance. This rule should only apply to heat sources that appreciably heat the bat beyond ambient temperatures.
Who knows what this official has been told, read or how he interprets it. Maybe some "higher up" in his association has instructed his umpires to rule like this. Maybe this umpire was just being one of those dreaded "overly-officious officials".
But it still sounds like a pretty nutty ruling to me.