So how does "any other" RDS radio work normally? I understand it can also switch and cut in to the music for an announcement.

The TMC channel is a data stream though. Whether it can be used in any way may not be possible though with out tying it to a navigation system.

From what I can gather it's essentially gives co-ordinates and speeds and or events. The nav system then needs to figure things out itself in terms of what it does with the info.