Playing with fog of war on, but periodically (in Diet sessions) revealing ownership of territories in the library seems a good compromise.

It can be a good surprise while playing as Chancellor to find out the hard way that an AI faction was suddenly where you did not expect it to be. E.g. it might have been fun to race for Antwerp but then discover the Danes had beaten us there.

But eventually word should get around and it would be silly to have an edict to take Antwerp premised on the wrong information about its owner.