This will probably my last word on this topic until I get a chance to bench-test these rules on Saturday's D&D game, but for now, a summary of how I intend to handle missile-fire generally and shots into mêlée in particular seems germane. So:
• I'm keeping missile ranges largely by-the-book, such that, e.g., daggers and hatchets are 10'/20'/30', darts and spears are 20'/40'/60', javelins are 30'/60'/90', slings are 40'/80'/120', hunting bows and harquebuses are 50'/100'/150', crossbows are 60'/120'/180', warbows are 70'/140'/210', and arbalests are 80'/160'/240'.
• Range modifiers are as per AD&D (±0/−2/−5) rather than OD&D (+1/±0/−1). As a general rule, long-range missile-fire is only possible where the ceiling is taller than close range for that weapon.
• Outdoors, the medium and long ranges can be read as yards rather than feet, but it's very difficult to hit a moving target at such extreme ranges: −11 at extreme medium range, and −23 at extreme long. This means, of course, that missile weapons actually have five range categories, e.g. 50'/100'/150'/300'/450' for a short bow.
• Shooting into a mêlée involves reckoning with both a cover penalty and a slightly increased range penalty: the shot is made at −1 if the target is flanked, −2 if the target is surrounded, and −3 if the target is obscured by a crowd; with an extra −1 at long range (or extreme range, for what it's worth). Whatever the net penalty (−1 to −4), this is also the chance that the missile strikes another target in the crowd or mêlée instead. If this happens (i.e. the initial attack roll was a miss because of the penalty), the actual target struck is determined randomly, with the chance that its armor is penetrated equal to 10% × the target's AC for a positive armor class (lessened by 1% per point of AC below 1).
That seems succinct enough to fit onto a page, at any rate.