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..!
tjclement
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I filed a report as well, because for obvious reasons completely disabling SIP is not an option on production machines. Bug ID FB8326129