I got one of Pioneer's CD-RB10 adapters and took it apart. This connects two RCA input jacks to Pioneer's IP-Bus with its differential audio signals. The adapter contains NOTHING. The RCA jacks are just wired to the audio signal wires in the IP-Bus connector.

The shields on the RCA connectors are isolated from the grounded metal enclosure. The installation guide says that the inputs should be limited to 1 volt. So I guess that answers my original question at least for the stock Pioneer bus.