The belt is a class-specific item available to mages, priests and warlocks. However, I have found that High Councillor's Sash has the same tones of purple, so it could work, although it's not as "angular" as the rest of the outfit, so I was glad to have this option. Also, Boots of the Malefic are also warlock-specific (and a pain to farm for), so you might just want to swap them out for Buccaneer's Boots which have the correct grey tone to them (not to mention you save a bank slot).
Featured above: Guldan's Shoulderpads of Triumph, Hallowed Garments, Handguards of Defiled Worlds, Ruthless Gladiator's Cord of Cruelty, Boots of the Malefic, Knife of Incision, Tome of Shadow Force.
Here is the same outfit, but with one of my favorite staffs: Abracadaver. I feel that the "spikiness" of the staff matches the shoulders perfectly. These two items both came out at the same time, so I imagine they were meant to match a bit.
Lastly, when I was using this outfit for T-mog, I also had Cape of the Black Baron equipped and displayed. It matches the shoulders perfectly and has a reddish-brownish-darkish color that fit as a cloak. Seeing as how the item is easy to farm, and I needed bank space, I sold it quite awhile back without realizing I didn't have any screenshots of it.
Well, apparently I had one, except it kind of hurts my reputation as being a successful warlock. /cough
Update Aug. 21st, 2012: I just remembered what cloak Anou was wearing in this shot: Cape of the Black Baron.