Yes, since sometime in the 7.9 series Ignition can read custom structures. The only caveat is that the server must support the OPC UA 1.03 and prior DataType Dictionary mechanism. Ignition does not yet support the new DataTypeDefinition attribute introduced in OPC UA 1.04.
I'm not sure now if I'm even trying to do the same thing as this original topic. We're trying to use the system.opcua.callMethod function, and we apparently need to pass in an ExtensionObject as a parameter to the function.
The code below on GitHub shows how to use Python to call the method, but it is using asyncua.ua.uatypes.ExtensionObject (e.g. line 37 & 38). I'm basically trying to find something similar in Ignition to put the data in the format that is required by the device when we call the method via Ignition.
It may or may not work depending on your Ignition version and whether the server supports the DataType Dictionary or not. Are there any structured values in the server you can try to read or subscribe to and find out?