Hm, I think the best compromise would be perhaps, allow it where there are "Magic Arrows" but it's attached to the bow you are using. Basically said Magic Arrow would act as a last resort when you run out of arrows without knowing it. As it stands now you can hold up to 999 arrows in one stack so I would think the aspect of holding onto that isn't so bad and if I"m not mistaken each CRP synth can make up to 333 at a time.

But basically as mentioned the magic arrows would use MP, so it isn't something that an Archer could fall back on and use all the time it would pretty much be situational where they run out of arrows without knowing but as to still stay useful in the battle, they could expend MP in order to still fight.

In terms of how strong they are I'd say perhaps they would just take the Archer's Bow into the attack equation +1DMG. While sure your dmg would suffer once you run out of arrows at least you'll still be able to fight mid-combat if you forgot to restock.