Sorry for bumping old thread but wanted to give my experience with GM LAN. I've poked and prodded around on a 2010 Camaro SS using Intrepid Vehicle Spy. I tapped into the HS CAN network by building an extension harness to go between the ECM connectors and the ECM itself. I'm definitely not a CAN expert by any means and really only know enough to be mildly dangerous but I was able to identify and reverse engineer all 19 of the ECM CAN messages and also identify many of the BCM messages as well. The scope of my project at the time was to replace the stock ECM with aftermarket EMS for performance tuning so I didn't mess with infotainment stuff at all. I was able to implement my aftermarket EMS into car and make the engine start and run and the car stayed happy even with the stock ECM completely removed from the car.
Anyways, my point in all this is that even though these other messages or channels may be on slower speed networks, I think they still propagate on the HS CAN. For instance, one of the things I needed to do was get all the stock gauges to work (tach, coolant temp, fuel level, trans temp, oil temp, and oil pressure). The gauges technically lived on LS CAN but everything coming out of the ECM/EMS is on HS CAN so there must be some carry over between the two. Unfortunately my knowledge and experience is too limited to fully know how exactly it works but I know that there are 39 other messages that are coming from other nodes besides the ECM.
Hope this helps in some way. I'd love to see the GM LAN stuff further developed.