Interesting, but the "outside" constraint processor issue could be a show-stopper. The "gradual tightening" of the DB is a key selling point of dynamic relational. Parts of it could be borrowed to make DR. And it's not clear how one queries it with SQL (or something very close). A translator layer would probably be needed.
It is a NoSQL Db, but has a lot of the features you're talking about. It doesn't have any form of "schema", but does allow for SQL style relational data. It definitely supports your "dynamic relational" data idea as stated above. The only issue is that any sort of data constraints need to be addressed at the outside of the Db itself.