Usb is a serial port, you should be able to use it:
https://www.kernel.org/doc/html/latest/admin-guide/serial-console.html
Of course, this is for generic linux, qubes’ dom0 is under xen, so you could try debugging xen too:
- Getting a serial console on USB-only Qubes laptop - this one seem to contain a good take on getting dom0 serial, but it requires a working system. It also contains information on getting xen console
- Is there a Serial Port Xen/Dom0 Debugging Tutorial? - this one focuses on xen console and only requires boot parameters, so can be used with grub access only
It also may cut your screen off to try and show a pretty image, there is a way to disable that to allow linux to display boot messages on screen, like this: