This is an insane question, I know. But it's one that's been bugging me for at least half my 33-year life. :)
I love both the Spelljammer and Ravenloft settings of AD&D 2nd Edition. One of the key tenets of Spelljammer was that you could "sail" (for lack of a better word) to any other D&D universe setting, or to any other setting of the DM's choosing, as long as you had a spellcaster capable of operating the helm.
So I always wondered, within the mythos of the Phlogiston and the spheres, could you Spelljam to Ravenloft? Did Ravenloft exist as a sphere, or was it only accessible when the mists opened?