I don't believe it is compeltely accurate to say that archers never fired into a melee that was already begun. Depending on how many troops were involved in a melee and how well trained the archers were, archers were able to target the back end of a unit of enemy troops and pick them off before they reached the engagement line, even if the front end of the troops in the unit are engaged unless they are very spread out, the entire unit wasn't always engaged at once.
Now of course while the full forces are fully engaged the archers usually wouldn't fire, but highly skilled archers could pick off individual targets from a large distance. I think allowing archers to fire into the back end of VERY LARGE melee should be allowed with reduced effectiveness for sure.
Perhaps also add an option for archers to retreat (without incurring casualties) once a melee is fully engaged. That way they can pull back before the fight reaches them and not lose soldiers. As it is archers aren't very effective unless on top of walls or by use of a very skilled commander.