GURPS would be a closer fit. You couldn't do Haroun properly with any
rules TSR ever published. And Mocker would be a mess of multi-classed
confusion.
How many points would having the Harish as an enemy be worth anyway ?
This would have been around the early days of second edition, but those rules wouldn't have been workable for the magic.
In most of Cook's worlds, there seems to:
Amateurs (who know a few tricks) - a level or two of a magic using class in D&D, or GURPS.
Average professions - who can make a living at magic - which pushes the envelop of GURPS.
Big Guns in Training - more talent and education than the average pro.
Big Gun - Say the Taken or the General Wizards of the Dread Empire
World Breakers - Lady, Dominator, and ... darn - I forget the name of the Empire Breaker (guy who took down the Empire of the West). It's on the tip of my tongue, and I'm sure I'll remember it as soon as I hit send.
GURPS can handle the professional wizard, but the basic magic system can't handle the "RUN! GET THE ARMY RUNNING! HE'S ON THE BATTLE FIELD!" style of wizardry of the big guns and beyond.
But anyway, TSR was in discussions with Glen Cook's agent about that, but it didn't come off. Not that game fiction is all that close to the games involved - reading most D&D novels, you have to wonder why this band of heroes can't find a 5th level cleric (could be wrong about that level, but well below 10th) to raise their dead friend. Game mechanics are routinely sacrificed for drama, and Dread Empire could have fit into some Forgotten Realms or Greyhawk clone.
Richard