thanks for your correction Jspira, I'm maybe confused but I hope you can help us with your knowledge
In reference to non Idrive systems, the CD drive build in the "Radio" (I'm not sure what exactly it called in BMW world) and not connected directly to the MOST bus via fiber.
Additionally, the problem is not the "MOST" as much as the way that BMW implemented it in close system type. That mean that the "master" computer that controls the MOST bus have to be programed to all of the active devices on the bus to let them communicate on the MOST bus. That is the main issue that if you adding a new device (like alarm or Ipod interface for example) you have to go to the dealer to "reprogram" the car.
Most of other cars manufactures using "open" protocol that have the capability to discover new devices (kind of plug-and-play) connected to MOST bus. Examples are VW, Audi, Saab, Citroen, Reno, etc...
I know that USB interface is available for some of those car already and I think that you are correct in the assumption that they are probably emulating CD changers and probably connected directly to the head unit and not to the Most bus (because of the issues that you mention earlier).
Sorry for long post, but I know that you have very technical posts and it looks that you have experience and access to BMW technical documentation.
Please advice if I got this wrong again