I am often curious when people think that early D&D was not anthropocentric.
As you say AD&D 1e and the Greyhawk folio and boxed set are humanocentric in the sense that humans are far more numerous than any other sentient being. I think this is also true of OD&D, Dave Arneson’s Blackmoor campaign, and the fantasy supplement for Chainmail. In their humanocentrism these worlds resemble Middle-Earth, the Hyborian Age, and Nehwon.
The world presented in B2 Keep on the Borderlands is different: "The Realm of mankind is narrow and constricted. Always the forces of Chaos press upon its borders, seeking to enslave its populace, rape its riches, and steal its treasures. If it were not for a stout few, many in the Realm would indeed fall prey to the evil which surrounds them."
It’s more like Poul Anderson's Three Hearts & Three Lions:
[H]umans were the chief agents on earth of Law, though most of them were so only unconsciously and some, witches and warlocks and evildoers, had sold out to Chaos. A few nonhuman beings also stood for Law. Ranged against them was almost the whole Middle World, which seemed to include realms like Faerie, Trollheim, and the Giants
"[T]he world of Law—of man—is hemmed in with strangeness, like an island in the sea of the Middle World."
The 1e DMG is opposed to monstrous PCs but OD&D is not:
There is no reason that players cannot be allowed to play as virtually anything, provided they begin relatively weak and work up to the top, i.e., a player wishing to be a Balrog would have to begin as let us say, a "young" one and progress upwards in the usual manner, steps being predetermined by the campaign referee.
Holmes Basic Set:
At the Dungeon Master's discretion a character can be anything his or her player wants him to be. Characters must always start out inexperienced and relatively weak and build on their experience. Thus, an expedition might include, in addition to the four basic classes and races (human, elven, dwarven, halflingish), a centaur, a lawful werebear, and a Japanese Samurai fighting man.
Mike Mornard played a balrog in Gary Gygax’s Greyhawk campaign.
Prominent monsters and other evil beings were also player characters in Arneson’s Blackmoor campaign. Jon Peterson, Playing at the World:
In Blackmoor, as it was played in the Twin Cities, most of the Baddies were nominally under the control of players; the orcs in the dungeon beneath Castle Blackmoor, for example, were answerable to Fred Funk (King Fred I of the Orcs) and the Wizard who lurked in its darkest recesses was played by John Soukop.
The infamous vampire Sir Fang played by David Fant is another example from Blackmoor.
Gary Gygax states that drow could be PCs as early as 1979. From the Sorcerer’s Scroll, Dragon #31: "The roles the various drow are designed to play in the series [the D1-3 modules] are commensurate with those of prospective player characters. In fact, the race could be used for player characters, providing that appropriate penalties were levied when a drow or half-drow was in the daylight world."