I've come across some bits and pieces that seem to be set on a Sorceror's Island, if not the Sorceror's Isle. The Crystal of Daerdaerdarth, one of the scenarios in the Stormbringer Companion, also included in the Games Workshop edition of the game, is set on a place called Earl Valyk's Isle; according to the scenario, this is located 80 km west of Sorceror's Isle. One of the scenarios in the Second Companion is entitled The Sorcerer's Isle, although if you look at the text it states that it is actually located on an island called Nisadnemepuur south of Melnibone proper. One of the main parts of Colin A. Brett's Golden Grimoire campaign is set on Sorceror's Isle, and includes a scanned hand-drawn map of the island and its immediate surrounding area; you can find it here:
Finally, there is a scenario entitled Madness and Other Colours by the redoubtable Lawrence Whitaker for Mythras, which you can find here and other places such as Drivethrurpg:
Reading between the lines, this one would appear to have originally been written for the Young Kingdoms, and the the sorcerer who made the dangerous statuettes, together with the so-called Ciirt short humanoids are very reminiscent of the spell thief Cran Liret and the Kretii. My own feeling is that this was intended for Sorceror's Isle, so that's where I'll be putting it when I run it for my lot...
Don't know if any of this helps, though!