The car is a 2012 Dodge Charger. I thought the dongle might actually have access to the CAN data even if it's not being used by most applications, but it looks like Chrysler vehicles use multiple independent CAN buses that probably don't communicate all the way back to the OBD port, so it looks like I'll have to tap into what they call the "CAN-B" bus to get the info I'm after.

There seem to be some folks who've done some similar projects in the registration-only area of canhack.org, so I'll probably pursue further lines of inquiry there. Was hoping it would be easier, but I guess I'm not surprised they'd firewall most of the car's systems off from the OBD connection.