I had looked at ORM Dynamic Models at some point, but they don't seem to fully cover what I have in mind.

This is more about the case where you don't know all attributes in advance and still want to pass them through in a generic fashion which I think is a common case in NoSQL, if not the primary motivation for using a schema-free store. Also I'm interested in the combination of fixed and unmapped attributes which I don't think is possible with Dynamic Models.