So it turns out that this particular issue was a silicon bug in the used NXP microcontroller (erratum ERR050101 - USB endpoint conflict) that caused its USB peripheral to lock up if any other USB device on a hub uses the same endpoint number as an isochronous endpoint on the microcontroller. Plugging the device directly into a host port fixed it immediately. However, it would still be great to know how to debug AppleUSBAudio for future reference..!