Even with the hand tucked into a shoe or shin guard strap, it's going to be exposed on nearly every swing, b/c human instinct is to bring the hands together. That hand is going to fly up and be hanging in mid-air, as much as she tries to keep it tucked away.
I don't agree with the "human instinct" part. Young players get taught to catch 2-handed right from the start. This is where the habit of bringing the hands together comes from (IMO). Just the same, catchers can be taught to catch one-handed and leave their hand protected by the ankle until the pitch is received. My DD has been doing it this way for years, in non-throwing/blocking situations.
I favor hand behind the mitt for throwing/blocking situations, and pulled back to the chest for receiving in those situations. Again, this has to be trained until it happens out of habit, every time.
We trained these areas relentlessly when my DD was beginning her journey as a catcher.