Hi Reinier,
I will try the tracing machanism to test the USB driver. Is there public the trace monitor version used to bring up the Rpi USB driver on base_hw?
I just went through my old branches and un-buried the attached patch.
Note that it is just a hack I developed for debugging a specific problem (the interaction of the kernel's scheduling with IRQ latencies of the USB driver). Even though it won't be immediately useful for your problem (I even doubt that the patch applies to the current version of Genode), it may still be useful for pointing you to the right places.
Cheers Norman