scott,
You second error message is closer....like you said, it means that you at least entered a valid serial port in the command. Now it's saying that it's not getting a response from the STM32. To use the FTDI chip on an Arduino as a USB-to-Serial converter to use with stm32loader.py, you'll need to make sure that the Arduino's processor is programmed so that it doesn't interfere with the communication. You could also either remove the Arduino's processor and use the FTDI chip alone (probably the easiest route) or write a sketch for the Arduino that makes it transparent.
I noticed that you said you were using the Audio Codec Shield. The example sketches for that shield disable Maple's virtual serial port (with the line "SerialUSB.end();" because the way the USB interrupts are serviced creates timing difficulties.