If you haven't already done so, ring the factory and talk to one of the fitters. They should know about linking to the PRO M-7 or contact someone in the office who does.
I am having excellent results with a Nokia 6230i paired to a 2820 using the Autocom special lead.
As the extra BMW buttons completely mask the location of the microphone input socket on the 2820, it has been deleted from the NavIII. This means that the Autocom cable solution will not work.