If we look to the descriptions of mass combat in Greyhawk, they don't read all that different from mass combat in Europe, and this is not surprising, given Gary Gygax's background in medieval wargaming and general interest in military history.
My thinking is that it should be able to get most of the "fantasy" units described in real-world terms. Orcs and hobgoblins are drilled, gnolls are not drilled (the former being lawful in alignment, the latter being chaotic), and matching armor and weapons should be easy enough. The Overking's Elite Guards are easy enough to classify in an army list, and overall parallels between the Flanaess and medieval Europe should be easy enough to make. The point being, there doesn't need to be a special set of rules just to deal with non-human troops. A few special exceptions may exist, but no more than are needed for real-world troop types (skeletons are only half as likely to be affected by swordsmen and the like, for example, and never have to check morale). Details such as armor class, hit dice, etc. are easily taken care of by using armor type, quality and training, etc.
That leaves the impact of specifically magical effects such as dragons, elementals, and high-powered spellcasters. That's a little trickier, but I think that delving back to the Chainmail Fantasy Supplement or Swords & Spells might prove beneficial. More when I have a chance to do a little digging.