I guess serial output works only if there is something at the default address of the serial port. The kernel does not know what USB is in the moment when its output begins. An USB to serial converter does this: It looks like a serial port to the other device but like USB to the system itself. You need it the other way round: It must look like a serial port to the local system, not matter what it looks like to the other device.
Hauke Laging
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