So there is no need to specify the baud rate with -b?
It was weird. After battling the OSX driver issue in my earlier attempt using the Arduino IDE on OSX, I was eventually able to get the USB passthrough working for the VM (resulting in the debug trace in the first post of this thread). For the attempt this past weekend I kept having a problem when attempting to enable passthrough for the device it gave me an error that the device was already in use (despite multiple host and VM reboots). I have since unloaded the CH340 driver and found an (unsigned) alternative that folks seem to have success with on Yosemite. If that also doesn’t work, I’ll try unloading that driver and just using USB passthrough to the VM.