I don't know if I'm just slow, but I finally realized why the Dungeon! game's combat system is so different from the man-to-man system. It's because it's the Fantasy combat system, extended to everything from skeletons and goblins up to purple worms and vampires. Not sure why it took me so long to see this. Or maybe I did before, but forgot about it.
I also got on Google last night, and read some people's reports of playing OD&D with Chainmail combat. Some were very positive, some very negative. Apparently, according to one account which claimed to be of a discussion with Rob Kuntz, Gygax never intended for anyone to play D&D with Chainmail combat except in the endgame when armies were clashing. The references to Chainmail, according to Kuntz as reported by a 3rd party, were just cross marketing.
Still, though, Arneson used Chainmail for his Blackmoor campaign, and Megarry used the Fantasy Combat rules for Dungeon! which was supposedly his take on how Dave A. ran Blackmoor. So I'm still curious as to how D&D with Chainmail combat would play. And I'd really just need to add the Cleric and Thief to the Fantasy combat table (maybe at high and low level values like the Hero/Superhero?).
Combat with mundane opponents would be Mass Combat by default, but breaking out Man-to-Man for special battles with human/humanoid opponents (but using D&D initiative instead of Chainmail's complicated version). When fantastic monsters are encountered, players could use Mass Combat or the Fantasy Combat table as they like.
Aside from the realization about Dungeon! just being the Fantasy Combat system, this morning I also thought about this idea. Any PC in the game will automatically get a squad of men-at-arms (or monster mercenaries if Chaotic?) with a Chainmail point value equal to their Charisma score. They would be able to add dice to Mass Combat rolls or help in Man-to-Man combat but not Fantasy combat, and absorb hits in any combat mode.