Since I can't leave things alone, I have gone through the process of figuring out how to retrofit the 1.0, 2.0, and now 3.0 DRVMs into my 4th gen. I have a Teensy 3.2 that serves the role of the BCM to send the mirror the correct LIN messages to function. Going to from the 2.0 to 3.0 setup allowed the mirror to show the rearview but it did not let me select the the side views (can't test the triple trailer view without a trailer camera).
@CortexM0 send me a LIN bus log from his truck since his has the side cameras enabled. I was able to get my mirror to show the side views after looking through the LIN messages his BCM was sending and modifying the code on my module accordingly. I am confident that the mirror gets the settings for camera setups from the BCM as I can change it from single view to single/double view with a couple parameter changes.
For anyone that plans to add the side cameras, settings changes in the BCM will be required to let the mirror know that there are side cameras present. I don't know which settings those are yet as I don't have a 2023 BCM to mess around with. It should be as simple as adding the cameras and changing the BCM setting once we figure out which BCM settings control the side cameras.