So I've really been digging into the core rulebook as I consider the possibility of transitioning my 4E group over to ACKS. I know, without a shadow of a doubt that certain characters are going to want to fire missiles into melee regardless of whether or not they have the Precise shooting proficiency. Basically, we play a game where if its something their character could conceivably try, we allow the attempt and adjudicate it best as we can. In this case, there's no physical reason an arrow or slingstone could not be loosed into the midst of melee, so saying "it's not allowed" would feel very jarring for us. So this is what I'm thinking (totally in the rough phase), please give feedback.
First, missile fire into melee is allowed with no restrictions when all participants in the melee are enemy combatants. A random roll based on the number of participants will decide who is the target and an attack throw will then be made as normal. (my players are notorious for inciting violence between two enemy groups)
Second, assuming a one on one melee between an enemy combatant and an ally, the missile attack may be made with a -4 to hit modifier as with precise shooting. However, a miss against the enemy combatant means an automatic hit against the ally (I want this to be a very risky maneuver with somewhat predictable negative results). Damage is rolled as normal for the weapon. The ally combatant is then allowed a saving throw vs. breath for half the rolled damage (rounded down). Any allies with the acrobatic proficiency gain a +2 to this saving throw.
These rules would also exist in the reverse, such that monsters could fire into melee while risking their allies to take a hit. Monsters of 4HD+ or higher whose main attacks were missile attacks would be assumed to have the precise shooter proficiency.