I have an SGI O2 w/dvi card and 1600 monitor. For some reason the monitor is VERY green. Before getting a replacement monitor or adapter card, I wanted to test them on a different machine.....
The special flatpanel card is a bit rare, so even if someone has an O2 it may or may not have the card.
If you hook up to the first serial port with the keyboard unplugged, what sort of text do you get?
O2's have a nasty habit of 'going green' and then 'going black' if there are RAM issues or another boot issue. Does the power light (on the O2) go amber, then after a bit go green, or does it stay amber (or worse, red)? My O2 has done this a few time; reseating the RAM fixed it once, and a replacement pair of RAM DIMMS (although one could call the O2's RAM Quad Inline Memory Modules since it has dual row staggered contacts) fixed it the second time. Some people on nekochan have had to reseat the CPU or do the 'jumpstart' pin jumper to get it to boot again.
Not meaning to be rude or to question your intelligence, but you've never pulled the mainboard while the AC power plug was plugged in to the power supply, right?
Have you tried a regular VGA monitor? You can't have both the 1600SW plugged into its special NON-DVI LVDS connector and a VGA plugged-in, but I think you can plug a regular VGA in while leaving the flatpanel card installed (I don't know that for sure, since I don't have the flatpanel card, even though I do have access to a 1600SW flatpanel at work (it's work's 1600SW, not mine, so I can't send it your way to test, sorry)).
Another good place to ask about SGI-specific issues is nekochan.net; are you a member there?