I messed with OpenFirmware a bit on a 6400 with linux. Not all OF implementations are created equal. Some of the stuff in the 6400 was broken and you couldn't get linux to boot straignt from OF. Newer machines don't have that problem.

iRCC, The 6400 even had enoughtfree programmable memory for short custom program. One I recall was to let you select a display modes other than the default. How many pc's do you see booting at resolutions above vga?

The 6400 only let you drop into OF during the post. Not like the Suns where stop-1a would take you the OF monitor mode anytime so long as it hadn't been passworded.