Turns out you can use the VISA Interactive Control (VISAic as we call it), I didn't realize it was there on the Mac as well as on the PC. VISAic will allow you to open a MAX-style VISA test panel for a COM port to see what you can read, and you can use
this post as a guide since it's very similar using VISAic and a MAX VISA test panel (starting around step 8, only you double-click the ASRLnn::INSTR entry instead of clicking an "Open Test Panel" button). If VISAic cannot see the USB-Serial converter or cannot communicate well with it, then you can guess where your problem lies...
VISAic can be found under Apps->National Instruments->VISA->"VISA Interactive Control".
-Danny