Well, Complete Mage has Fey Presence:
You share your ancestors' knack for playing tricks on the minds of others.
Fey Heritage, character level 6th, Nonlawful alignment
You gain the following spell-like abilities, each usable once per day: charm monster, deep slumber, and disguise self. Your caster level equals your character level.
Obvious problems include weird fey influence on your devil, and alignment issues. Both things I’m inclined to hand-wave (for players as well as NPCs), but they are there. Also requires Fey Heritage, which is basically garbage (+3 Will vs. Enchantments). I’ll keep looking.
Aberrant Dragonmark (Player’s Guide to Eberron) works for certain (Humanoid) races but not for an Osyluth (or much of anything else that qualifies for Fiend of Corruption, though I suppose a Half-fiend Human/Dwarf/Elf/Halfling/Orc could do it).
I’m pretty sure that’s it for feats.
For templates, Half-fey (Fiend Folio, same as Fiend of Corruption in the first place) is CR +1, gives charm person at will, and a bunch of other stuff that’s pretty useful (like flight). Of course, if you’re going for something that’ll increase CR, Beguiler (Player’s Handbook II), Dragonfire Adept (Dragon Magic), Sorcerer, Warlock (Complete Arcane), and Wizard all work, and I think Binder (Tome of Magic) does too.