Further testing has made it apparent that the tool can, in fact, read from the COM port. Making it so that the machine sends data continuously, then plugging in the tool in the middle of the exchange, makes it able to read the data, as long as it is sent. The exception happens precisely when no data is being sent.

I'm still stumped as to why an exception would be thrown on certain environments and not on others, but at the very least the data's going through, so I can work with that.

I've made a "band-aid" solution by simply catching and ignore this specific exception, using the HResult property of the exception to separate it from other unrelated IOExceptions.