Bob,
Where in the rules is that published?
I looked and didn't see it posted/ spelled out in the rules, but I'm not sure if my rule book has all the latest changes, as it's an older book and I know there have been quite a few changes since then. Keep in mind a 'trigger shoe' is not an offering on the original factory rifle, so it's all in how you, or the officials, apply the rules.
What I do know for a fact is that a friend I shoot with had to take his trigger shoe off at the Nationals for them to let him shoot his rifle in hunter class (Remington 541), but that was a few years ago.
That's why I said to send a note to Greg (NRA) and get it in writting. It's no fun practicing/shooting all year one way only to find out you have to change something when you get to a big match, then nothing 'feels' right and your whole day is ruined.