ARM is now on the table, as I am bridging the coreboot console from USB to gigabit ethernet, using a BeagleBone board equipped with the USB debug device gadget driver. At first I was a bit concerned of all the latency having an USB-to-ethernet bridge software solution on the communication, so it was now good time to do some measurements.
I used daemon called ser2net to redirect coreboot console TTY to TCP (telnet) and enabled timestamping to estimate the time it takes from power-on to entering payload on amd/persimmon with maximum logging (level=spew).
First, connecting with serial UART @115200bps on x86 host this was total of 15 seconds. Of this it spent 4 seconds in AGESA doing some SPI flash operations and during that time there was no console output.
Repeating same using usbdebug on the BeagleBone OTG port, total time was 9 seconds and again 4 seconds was waiting for SPI completion. I would say we have a rough figure that console output on usbdebug is twice as fast as what super-IO can typically offer.
Driver compilation and patches are updated here: http://www.coreboot.org/EHCI_Gadget_Debug
To use BeagleBone some additional work was needed on coreboot side but BeagleBone Black and other ARM boards where there is no hub between OTG connector and controller should already work.