Assumptions: all material from spelljammer, 3.0, 3.5, web/dragon and Pathfinder are available. Everything in this campaign setting is being converted to Pathfinder. All available separate campaign settings are their own crystal spheres. I try to remove as much DM fiat as I can, but some is required to do conversions and a lot is required to implement spelljammer, which this question is based around.
This section will be updated based on clarification request. Please comment to suggest clarifications.
Problem: Inter-planar travel is not possible outside of a crystal sphere. The dimension of dreams / region of dreams exist as kinda sort of part of the ethereal plane. If you can dream, you go to those places as a mental projection and play around in your dream demiplane. Some crystal spheres have more danger here than others depending on local creatures.
If you're outside of the spheres however Inter-planar spells and abilities don't work. Can you still enter the Dream world via sleeping? If the answer is yes, can you do so with spells/powers/abilities? Are there Night Hags prowling the spaces between spheres through the dreams of creatures traveling in the phlogiston?