I think that there's a mix up in information here. OBD2 is a standard
that was adopted by all manufacturers for cars intended for the US market in 1996. All of these cars have an OBD2 diagnostic port that will provide information such as emission level DTC's. As it was not required for cars outside the US, many manufacturers left them out until 2001 when Europe adopted a similar standard (EOBD). Many cars that had an OBD2 socket also had a proprietary diagnostic socket that would allow them to access other non emissions related systems.
Many people call these proprietary connectors OBD1, This is not really a standard, but more of a requirement
originating from California for cars to have onboard diagnostics. Different manufacturers adopted different standards and there was a rarely a generic tool that would cover them all. Some cars like Jaguar did not require a tool as there was a way to display trouble codes on the same LCD screen that they used for the clock.
The RR and Bentley SZ model cars from 1992 onwards had a round diagnostic connector to access codes and live data. In 1996, they added an OBD2 connector next to the round plug for cars bound for the USA. The information from that port was limited to emissions related codes and live data (like 02 sensor readings).
In 1999 when they introduced the Arnage and Sereph, they did away with the round diagnostic socket and used the OBD2 diagnostic socket as its only diagnostic socket. As OBD2 only uses 7 of the 16 pins for emissions related codes. They could still use the remaining pins for their other systems. To access all of the systems, you will need a proprietary Bentley scan tool and cable which is programmed to access those systems.
From my experience (and I beat this thing to death trying to figure out transmission and alarm issues on the Brooklands and the Arnage), unless you have the OMITEC scan tool, you cannot access all
of the systems (climate control, alarm, suspension....). That's because they're not a CAN bus. I was able to access many of the engine systems on the Arnage using my Autel scan tool and read the live data through the generic OBD2 menu option. The autel does havea specific Bentley Arnange menu but only for models 2006 and later. I assume that the car had a major update at that time and adopted CAN which was a standard that was going to be required for all models after 2008.
In any case, the SZ cars with Zytec supported OBD2 in a limited fashion like many cars did from 1996, so if they reverted back to the very same ECU's used on the SZ cars, it would have supported OBD2 in the way that I described.