I don't believe there is that much intelligence in the NAV DVD/CD rom unit in relative terms. IE: 10 years ago yes but today no. The IBUS communications is well documented as its been around for quite a long time. It would actually be less work and less invasive to emulate a NAV unit than one part of it. As mentioned it would involve reverse engineering. IE: The NAV unit turns on and says I am here. It states what it is and is polled for information and responds to information. The communications is similar but not NMEA sentences. In the US typically a company will apply for a patent for technology and many times this patent well documents what the technology is / methodology used for said technology. As VDO was "multitasking" the same / similar use of the base standard that they created was utilized as a one for all. So somewhere there is a list that BMW provided its vendors for the base for IBUS communications devices. Its probably as easy as a predefined base of communications standards that any device can use. I remember once testing a MKI TV module on my E46 and noticed that the "intelligence" was in that device rather than the NAV CD device and it "took over" the intelligence on the display. I installed a "new" device on my E83 called a CIS-IBUS interface. This device actually has its own display on the 16:9 NAV display. Just like another device on the bus. (IE: hitting the mod button will bring up the CIS-IBUS interface).

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E83 - Aero PKG Mods installed: Homelink mirror. CIS-IBUS video module, AUX, Rr Dr Release, stock wheels, Remote start, CarPC (Atom) & mud flaps - NAV - MKIV - NA Map DVD - 2011
E46 - 18.5 wheels/tires (summer), 17 stock (winter), Homelink, NAV software upgrade, BMW video module, Radio to V53, AUX, USB IBUS interface,BMW alarm install, CarPC(Epia), remote starter, tank battery & angel eyes. NAV - MKIII - NA Map CD - High Version 2008.1