I'm making small progress with the graphics card problem. I've looked into the DSDT source code again and I think the key to switching the graphics adapters is a device called "WMI1", which has the PNP id "pnp0c14". I discovered that already a few days ago but when I tried talking to the device I could just not make the acpi driver register to it. Yesterday I was searching around in the acpi kernel drivers and I found a driver module "wmi.ko" which registers a driver to the very same PNP id. However, this driver doesn't do much on the Z21 but hogging the device. It's registering an event notifier and tries to find an embedded controller area, which is not present on the Z21 and so it does nothing useful. I think there's a DMI match missing, it should not register drivers blocking access to certain devices when it's unable to do anything useful with them.
(Update: After some more research I found that "WMI" reads as "Windows Management Instrumentation". wmi.ko is a wrapper driver allowing access to the WMI functions through ACPI. Now Sony seems to use, or rather abuse, the WMI interface to do other device specific functions as well.)
Walking the namespace of the "WMI1" device reveals that there is just one method, "WMMX", which basically acts as a wrapper to access the functions of both built in graphics adapters, "OVGA", the Intel GM45, and "DGPU", the Nvidia, and also methods of their respective output devices, "LCD", "CRT" and "DVI". I took a deep dive into the DSDT listing again and identified a couple of interesting things to try next. It's all very fascinating ...
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