I figured it out. The answer is manufacturer dependent. The OBD-II diagnostic port has a number of pins which are listed as "Vendor Option". The manufacturer may choose to connect one or more of these pins directly to the Medium Speed CAN Bus or the Low Speed CAN Bus. Doing so will provide direct access to message traffic along those Bus lines in real time. If a gateway is involved there are two basic approaches.
A) This is the more popular option. The OBD-II port connects directly to the High speed CAN bus. The port can directly monitor traffic along the High Speed CAN bus. One of the nodes on the HS CAN bus (typically the Body Control Module or something similar) acts as the gateway to the Medium Speed CAN bus. To access data on the MS bus, a diagnostic Remote Frame (IAW CAN Spec 2.0) is transmitted on the HS bus and meant for the gateway node. Upon reception the gateway node then generates an additional Remote Frame on the MS bus meant for whatever the target node on that bus is. Upon reception the target node then transmits the requested data on the MS bus. The gateway node receives it, then when the HS bus is idle it will transmit that data across the HS bus.
B) The OBD-II port may be connected to a dedicated gateway which will only deliver data from a specific Bus if a diagnostic request is sent through the port. If no request is sent there will be no discernible traffic. You must always send a diagnostic remote frame request message if you want to pull data from a particular node on a particular bus.