Using the proprietary firmware for this would be ideal. It would also
provide reliable access to the kernel debugger which would be extremely
helpful for diagnosing what's going wrong with the console. I'm not
sure how I would go about making progress on this though. I know there
are binaries of the BIOS for Sun4m machines floating around but I'm not
aware of any for Sun4u machines.
I haven't been able to find any of this firmware either. Not sure if this helps but someone says they've got the Ultra 1 firmware (along with cgsix and cgthree) available here: