Hello.
I think that the second link you provided (
www.progea.com/forums/topic/opc-ua-dynamic-addressing/ , after correction), refers to a different concept altogether.
However, I think that you are talking about the same thing that I described - that is, from perspective of the client, there are items/nodes that may not be visible/browseable in the address space of the OPC server, and that are effectively created by the simple fact that the client uses them in reads/writes/subscriptions, provided that the string ItemID/NodeId conforms to some server-defined syntax from which the server can parse out the register address, or element index, or whatever is needed to access the data.
This should be perfectly well supported by QuickOPC.
You wrote "I keep getting a BadUnexpectedError when the EasyUAClient tries to write to them, even though the DA client that comes with the KepWare could dynamically write to them without problem. " Well, there is almost no conclusion that can be drawn from this test, because you used OPC DA communication in one test, and OPC UA in the other. I suggest that you try some other OPC UA client - such as the popular UaExpert, to see if you can write over OPC UA with it. If you can do it with some client, I would be more than willing to investigate why QuickOPC fails in the same operation.
Best regards