AllowWeaponTargetCheck(attUnitID, attWeaponNum, attWeaponDefID) --> true | false
AllowWeaponTarget(attUnitID, tgtUnitID, attWeaponNum, attWeaponDefID) --> true | false [, tgtPriority]

Engine:SlowUpdate()
    for every unit u
        for every weapon w of u
            if Lua:AllowWeaponTargetCheck(u, w, ...)
                for each potential target unit t in range of w
                    allowed, priority = Lua:AllowWeaponTarget(u, t, w, ...)
                    if allowed
                        insert t into target list of w with returned priority
                    else
                        -- this is subject to change (I may have it skip t
                        -- entirely if the callin exists but returns false,
                        -- as opposed to when it doesn't exist at all)
                        calculate priority value via hardcoded logic
                        insert t into target list of w with that value

                sort target list by priority (increasing order)
                for each target t in list
                    if t can be hit by w
                        select t as target for w; break