There is a minor notation of where Icosiol's tomb is geographically located. AGoW gives this information, "Icosiol's tomb is in fact deep under the ancient Fields of Pesh, in a region about 500 miles northeast of Diamond Lake. If the PC's somehow manage to travel up from the tomb through 1000' of bedrock, they emerge in a barren stretch of plains. To the north a single volcano looms, a plume of steam drifting from its peak." (White Plume Mountain)
Near as I can figure it is about 450 miles to White Plume Mountain from Diamond Lake (Greyhawk poster map). As the intent was to have them south of the mountain, did anyone work out how far north of Alhaster this "barren stretch of plains" would be? Secondly, are there any recommendations on whether I treat it as Shield Lands area, or do I involve some of the more nefarious elements of the Rift Canyon? (this is where I'm leaning towards...)
As you note, the information relating to the tomb's exact location is only approximate, but I'd say it's closer to White Plume Mountain than Alhaster. The area surrounding White Plume Mountain, and the Fields of Pesh, is pretty barren by all accounts.
Given the set-up of this adventure, if your party somehow surfaces from the underdark, say by dimension door spell or something similar, you can pretty much put it wherever you want it. In practical terms, there's not much difference between the Shield Lands and the Bandit Kingdoms these days--they're both overrun by Iuz's forces. White Plume Mountain is technically in the Bandit Kingdoms, but probably visible from the borders of the Shield Lands, and that whole section of the map is pretty desolate these days anyway, so who claims the land is not very important. If you want to use this as a foretaste of what the PCs will encounter later, by all means set them closer to the Rift and throw a couple of appropriate encounters at them. Dragon #276 has some additional info on the Wormcrawl Fissure and a couple of colorful monsters that could serve as random encounters.
So, some days it's extremely useful to have several in my group that have been through many of the original published adventures. I told them of the volcanic activity to the North (I put them 20 miles south of the mountain). Once they sorted through that a solitary volcano wouldn't put them in the Hellfire Furnaces, they put a name to it (Hooray for player knowledge that can be backed up by PC knowledge skills!) and went into spasms--blubbering about the wizard and Dragotha. After these two explained it to the less experienced members, they quickly decided they didn't want to be there and chose another plan.
Thank you for your help and suggestions!