I heard a rumor that there is a published reference to a layer of the Abyss that is controlled/populated by celestials. Is this true? If so, what source mentions this?
I'm really just curious about such an oddity on the planes.
The Forgotten Realms wiki on Pale Night now gives information directly relevant to your question.
The following is listed under 'Realm':
Despite being a surreal realm of endless war, Androlynne was one of the most pleasant places within the Abyss since it was defined not by the conflict of evil versus itself but by the struggle of evil to fend off the forces of good. Mother's Mountain, Pale Night's occasional, looming residence, was a reminder of Androlynne's original identity that she helped keep intact, a bleak, colorless realm of misery and nightmares. However, the rest of the layer was a beacon of hope for what it represented, the power of good to change a place even as hopelessly corrupt as the depths of the Abyss. As a result of the vast number of benevolent beings residing there, ranging from chaotic eladrins and foo creatures, to the more neutral moon dogs and hollyphants, to lawful ki-rins and couatls, most of Androlynne became a beautiful land of bright pastel colors and rolling hills. The vivid fields were rich with flowers and other alien vegetation and the sky housed light-purple clouds of unimaginable size that sometimes took the shapes of screaming faces and at others resembled animals or random household items.
Paragraphs above and below link to this material: Ed Stark, James Jacobs, Erik Mona (June 13, 2006). Fiendish Codex I: Hordes of the Abyss. (Wizards of the Coast), pp. 148–155. ISBN 0-7869-3919-2.
The following quote is listed under 'History':
The kidnapping of the innocent children prompted celestials and other goodly creatures from across the Upper Planes to protect them, as over time an all-out war broke-out in Androlynne. Ascodel eventually died protecting the eladrin children, whose numbers were somewhere in the double digits, but their rarity only made their defenders fight more rigorously to protect those that remained and the successor Royal Consort, Faerinaal, spent much of his time personally overseeing the conflict. The pact had both bound the eladrin to Androlynne and kept them eternally young, and the celestial defenders worked tirelessly to break Pale Night's hold over them, their centuries of struggle, majority influence, and the childrens' own purity evidenced by Androlynne's new appearance.
You wanted confirmation that these creatures were chaotic evil. If this is canon, there is specific mention that the ki-rin and coatl are lawful.
On page 62 of the DM's Guide states the (optional) Abyssal Corruption only happens on a failed DC10 charisma save. This corruption is always removable via Dispel Good / Evil magic. The point: properly equipped celestials would-could easily support one another's alignments in The Abyss.
Note: Your discussion regarding this was seven years ago. It is possible that the campaign relevant to this question may have already started (scheduling permitting / your results may vary).
It's from a 2e Planescape supplement called Faces of Evil: The Fiends, and while it doesn't appear to be a whole layer, it is all data I can find on official stuff existing on that layer.
In short, on the 403rd layer of the Abyss, a Fallen Archon rules the City of Confusion, and several other Fallen Archons are her minions. They try to cure the mortals that come across the city, purify their souls, so they can go to Celestia. They fail at it, however, because these Fallen Archons have become as Chaotic Evil as the plane they live on. They are just in denial about this.
After hints from @Lord_Gareth and a fair bit of searching, I found what I was looking for: Androlynne, Abyssal Layer 471. It is detailed on page 148 of the D&D 3.5 Fiendish Codex I - Hordes of the Abyss.
It is ruled by an Obyrith, Pale Night, so it isn't really controlled by Celestials. Eladrin children are bound here, and Good creatures wage a never-ending war on this Layer in order to rescue and/or defend them. The Good creatures have maintained such a presence on this layer that it has been transformed from a nightmare landscape to a bright rainbow-hued battleground.