So here's the solution to that, and, I think, a much more stable and trustworthy measurement of what I'm calling Equivalent Hit Dice (EHD), which can be used to gauge threat levels and apply simple XP awards without complicated table look-ups (e.g., as in the original system awarding 100 XP per HD). As of version 1.08 of the Arena simulator, I implemented Party-on-Party battles, so that arbitrary numbers of men or monsters could face off against each other and get a view of data on those scenarios.
Before I present the output from that Party-based simulator (that will be next time), a few thoughts on the model of combat that went into it. First: exactly who attacks who? In the AD&D DMG, Gygax gives recommendations which fully abstract the selection, reflecting the fact that his games had removed miniatures from the table by that point:
As with missile fire, it is generally not possible to select a specific opponent in a mass melee. If this is the case, simply use some random number generation to find out which attacks are upon which opponents, remembering that only a certain number of attacks can usually be made upon one opponent... (AD&D DMG, p. 70)
That seemed like a pretty handy model to implement, because it meant that I didn't have to track actual position or movement of any of our simulated combatants. In fact, I went so far as to randomize the target of every separate blow, which was a pretty simple way to parse out multiple attacks (e.g., as for those creatures who can battle multiple opponents). Furthermore, I set a maximum number of attacks against any creature in a round at 6 (again, taking a cue from the hex-based rule in DMG p. 69; and didn't bother to adjust for relative sizes). Note that a side-effect of this is to come close to coincidentally implementing the AD&D rule for Hydras that "up to 4 heads are able to attack the same target simultaneously" (while not going quite so far with the safety bumper, this will reduce the immense number of attacks that made Hydras look so deadly in the prior model against just one target at a time). Another side-effect is that there will possibly be some small bit of bias for mobs of many creatures, as one can possibly take a hit and keeping fighting as other targets take their place (as opposed to a fighter picking one target and attacking until that one goes down from the fight).
The Party-based combat also made it worthwhile to implement a number of special abilities that had to be left out previously, notably area-attacks such as: Breath Weapons, Petrifying Gaze, Manticore Tail Spikes (shot among many opponents), Vampire Summoning, and Dragon Fear. With each of these special attacks I assume that the monster in question automatically gets one "entry attack" usage before melee begins. For example: With Petrifying Gaze (e.g., Basilisks and Medusae), the entire opposing party all have to make initial saves or turn to stone at the outset; thereafter, I assume everyone knows to "avert their gaze", and the melee proceeds with everyone getting −4 to hit the monster, but no further gaze attacks are assessed. Breath Weapons are abstractedly assumed to hit about half the opposing party (meleers on one side?), or a maximum number scaled to the breath area (4 for Chimera, 7 for Gorgons, 14 for Dragons). Fear from Dragons (included as part of the description from Chainmail) was assessed as a D&D morale/loyalty roll; all opponents must roll 2d6 + HD ≥ 6 or else flee before the fight starts.
Discuss these rulings for mass melee: reasonable or unreasonable? Which of the special abilities mentioned above would you guess are the most dangerous?