Hello Mabons,
I'm not sure about this, I don't play with the "face to enemy" (F2, F2) in captain when I am placing a shield wall. I can assure you that a solo guy (a player) can run back to your shield wall and single-handedly attract the attention of your infantry. It means that the shield wall don't watch anymore the archers. They just turn their backs to face the "one man army".
It's hard to make a short video clip for the moment, but if we could, in the future, play the captain mod in private server, it will be easy for me to show you a visual exemple.
It's like in a siege in the single player campaign. Defenders face the gate, but if you run to the back-line you will see 4 or 5 soldiers turn their backs.