My thought is that it should be difficult to even get a shot off. Per HM3 -
A missile may be fired at a target engaged in melee. However, a “Miss” has a 50%
chance of striking a randomly-determined adjacent character.
I would think a "Miss" means anything that does not strike the target or a shield.
From HMG (which uses different a slightly different missile system) it does not seem to specify the actions of firing into a melee, but we have this:
If the GM feels all or part of the target is obscured s/he may declare some boxes on the strike location table ‘covered’. If, for example, the target is standing behind a low
wall, the GM may declare rows 1-13 to be ‘covered’. Thereafter, if the missile strikes in any of these covered boxes, it has struck the wall.
A diagram is superimposed on the humanoid target showing the relative shapes/sizes of a
buckler, round shield, knight’s shield, kite shield and tower shield. This is meant as a guide for the GM for targets carrying shields. The GM is free to designate these outlines as being somewhere else on the matrix, and may, discard them completely. In some cases the GM may designate one or more boxes as ‘partially covered’. In the event that the
missile strikes one of these boxes, the GM may require an additional die roll to determine whether or not the missile strikes the obstruction.
The attacker declares the Target at which the missile will be aimed. it must be within range of the missile weapon and in line of sight.
From this (which is perhaps open to some interpretation) first question is line of sight - which perhaps means the LOS is obscured if there is a target engaged in melee, so no loosing of arrows.
Otherwise, if interpreted to allow missile fire into the hex, one must determine where exactly the missile strikes, where the "friendlies" are as to LOS - a very difficult/complicated process.
IMO, the HM3 system makes loosing arrows into a melee to easy, HMG makes it too complicated.
I've used some experimental "houserules" for this as well. This is one method I have used:
1) The missile user rolls either Ini, Missile ML, or Awareness (Or a composite of the above). The base idea for the roll is the ML mentioned, using the following formula:
This is the first roll. a CS/MS means they may then losse into combat, a MF/CF means they must pass without being able to loose into the melee.
The next step is then to loose into Melee, here I'd use the HM3 system for loosing into a melee.