I have been reading the Metabox Docs on Custom models and am surprised that it does not list MB Relationships as a compatible extension.
If there is a technical reason for this omission, my suggestion would be an additional field in the MB Relationship table that stores the Post/Model name.
With some accompanying code, this would additionally enable any relationship combination - i.e. between Post-Model, Page-Model or Model-Model.
What are your thoughts?