I wonder if Zariel is mentioned in any of the old "Politics of Hell" articles from Dragon magazine, waaaaaaaaaaay back in the day...?
EDIT: Issue #28! From 1979! But not the one I was thinking of. Further research reveals: "Dozens of unique devils appeared in a two-part article by Ed Greenwood, including the greater devils Bist, Caim, and Nergal, the dukes of Hell Agares, Alocer, Amduscias, Arioch, Balan, Bathym, Biffant, Caarcrinolaas, Chamo, Focalor, Gaziel, Gorson, Herodias, Machalas, Malphas, Melchon, and Merodach, and the princesses of Hell Cozbi, Lilis, and Naome in 'The Nine Hells Part I' (Dragon #75) and the dukes of Hell Abigor, Adonides, Barbas, Barbatos, Bele, Bifrons, Bileth, Buer, Bune, Morax, Neabaz, Rimmon, Tartach, Zagum, and Zepar, the princesses of Hell Baalphegor, Baftis, and Lilith, the chancellor of Hell Adramalech, the queen of Hell Bensozia, and the inquisitor of Hell Phongor in 'The Nine Hells Part II' (Dragon #76 (August 1983). Ed Greenwood's follow-up article, "The Nine Hells Revisited" in Dragon #91 (November 1984) introduced the greater devils Armaros, Azazel, Cahor, Dagon, Duskur, Kochbiel, Malarea, Nisroch, Rumjal, and the arch-devil Gargoth." No sign of Zariel :(
Zariel is strictly from late second edition post Planescape. She was mentioned first but unnamed in a Dragon Magazine article that re-introduce the Arch-devils. She was given the name Zariel later in Guide To Hell (I think, could be wrong).