Ok, so I've spent some time today with the cluster in hand and on Sedre.
Picture of the cluster:
Picture of the back, showing the large matrix connector, which has six (6) pins.
I've compared the wiring diagrams of my 308 (small display) with the RCZ that had the cluster with the large one.
According to my notes, the connector for the large display has the following pins:
-Pin 1: +12V feed from the BSI
-Pin 2: CAN Low
-Pin 3: CAN High
-Pin 6: Ground connection, shared with the ground of the COMM 2000 (SCCM, Steering Column Control Module)
The interesting part is that Pins 4 and 5 seem to be allocated to the switches that are placed next to the handbrake, which are not there in this car. I've confirmed that from Servicebox.
Some more thoughts of mine, as my job is on CAN:
The cluster with the small display has no hardwired connection to the SCCM. It is connected to the BSI using the Comfort CANbus. The SCCM is connected to the BSI using the Body CANbus. Therefore, the action of pressing the button at the end of the wipers stalk is transmitted from the SCCM to the BSI and then to the cluster, and the various trip computer screens are accessed. I would say that the same behaviour should be expected from the large screen.
Therefore, what is needed is to splice into the two CAN wires that go to the cluster anyway and connect the power and ground feeds.
The biggest challenge is getting my hands on the actual connector that is needed.