I'm going to wish you luck here. I should point you to this thread between "Kumba" and myself over on the Gentoo Mips list from days long past.
In short, someone did get basic functionality, but wouldn't release his work for anyone to see. I tried nicely to contact the guy a few times myself. No dice.
--- On Tue, 11/25/08, Dmitri Vorobiev <email@example.com>
From: Dmitri Vorobiev <firstname.lastname@example.org>
Subject: Looking for Extreme graphics technical documentation
Date: Tuesday, November 25, 2008, 5:49
I am looking for Extreme graphics technical documentation. Indigo2
PROM reports the graphics board as follows:
controller display SGI-GU1-Extreme key 0 ( ConsoleOut Output )
peripheral monitor key 0
So far I have been able to find (apart from a handful of marketing
documents) only the following two sources:
These, however, do not seem to be enough to write a console driver.
Can anyone help, please?