I tend not to respond when I have no idea

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Random somewhat related comments:
* Patching diags to get SSH in there and make it stick is kind of the RescuePack's sole purpose in life

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* I don't think anyone ever attempted anything related to USBSerial (I'm not even sure the kernels support that gadget). So, yeah, monitoring would be far more reliable over serial.
If it does boot, the kernel logs are somewhat sanely archived by the logging system, although this sometimes goes a bit wonky.