+1 As a crossbow enthusiast, I would definitely like that.
Archers have the ability to move while reloading, making it extremely hard to duel them with a crossbow if you don't have cover yourself and the archer can aim/predict at least decently. Also, in BL you can shoot relatively accurately while moving thanks to some of the archer perks. Having a deployable shield isn't anything too crazy.
The Mercenaries MP mod did this in Warband (and possibly/likely cRPG, can't remember). It wasn't broken at all or anything. Just some flavor to playing crossbowman and a way to make them not entirely useless in long range, open field engagements.
Another way to do this could be that the reload animation is changed while having a pavise shield. The crossbowman would turn around and crouch while reloading, while having the shield on his back towards the enemy. Alternatively, the animation would be tweaked only very little, so that the shield on the back moves to a good spot for blocking arrows, while the player has to turn away from the enemy himself if he wants to block the arrows. This would be a fun, unique way to play a crossbowman. Bannerlord even has this system where the shield blocks some hits when it's on your back.
I've even tried how well it works for arrows, but it's not too strong and will interrupt the reloading animation. Giving it a small buff so it will take the full hits, or for balance, drop the damage from arrows to something low enough so it doesn't stagger. This would allow for protection from some arrows from long range without giving you god mode vs. say, 5 archers constantly hammering you with shots from near you.