Back in 3.x,(in I think a Forgotten Realms supplement) there was a spell that allowed you to apply the half-fiend template to a creature. I think it was usually a willing creature, but I believe it could also be applied to unwilling ones.
What would happen if this was used on a celestial? I'm pretty sure there was no restriction on what sort of creature it could be used on, save perhaps for the subject couldn't already be a half-fiend.
I could see evil outsiders using this as a torture method for captured celestials. I could also see a reverse version(adding the half-celestial template) being used as an extreme conversion/redemption tool by some of more hardcore among the good hosts.
Not quite. Going through all of the 3.5 stuff I could find, the only thing I could find referencing half-___ templates for spells (Including Faerun stuff) was the Visage of the Deity line from Complete Divine, of which the Greater Visage of the Deity spell turned you into a half-celestial or half-fiend, but it's a 9th level spell and personal only.
There in an occult ritual that temporarily grants the half-fiend template in Pathfinder Campaign Setting: Occult Realms. It only affects the one performing the ritual and only lasts one week, but a different version could affect another (unwilling) person and last longer.
Vile Death from Heroes of Horror, applies the fiendish template to an undead creature.