Our next firmware release (which should be available within a week) will include updates to the UART firmware example.
Another option is that if you don't want to use the firmware functionality at all, you can 'turn it off' and communicate directly to the modem instead by using the hologram_dash_passthough.ino sketch in DashExamples from the Arduino IDE. That will send all communication from the USB serial port to/from the modem.
Pins L11 and L12 can be used to access the internal CAN Controller on the MCU. We don't provide any drivers for it, but it is documented in the MCU Reference Manual in Chapter 46. NXP/Freescale does provide drivers that could be used to build your application. As that is only the Controller portion, you'd also need to provide a CAN transceiver part that converts TX/RX to the physical CAN bus.
Since you have to add a component anyway, another option is a combination CAN transceiver/controller part that would interface to the Dash using SPI instead of using the internal CAN controller.