I came across this issue as well, and I've spent nearly 2 hours online searching for solutions. You were on the right track, yaboot is the right place to look at. However, there are some more to do before we get rid of that useless monitor.

I found an email conversation, which resolves our problem. I am not too sure if you've resolved yours, but for the benefit of others, I will repost the solution and link here. (Just because Google thinks there is a solution here...)

Link: http://comments.gmane.org/gmane.linux.ports.ppc.embedded/34058

In case the link expires, here is the copy-and-paste:

Credit goes to: Romain Goyet wrote on 24 Mar 2010 11:32

Hi there,



  Here's a summary about "how to boot a PowerMac G5 without a screen

attached". As many people have noticed, default yaboot install won't

boot unless a screen is attached.



  Actually, the workaround is really simple. Thing is, that's the

"ofboot.b" script that messes things up. This script is a "bridge",

that actually either starts yaboot or let you boot off of a CD. I

don't know what's the point, since OpenFirmware already lets you boot

off of the CD by pressing C on boot…



  So basically, to boot headless, all you have to do is to tell

OpenFirmware to boot straight off the yaboot binary, and to skip the

ofboot.b

Just boot into OpenFirmware (Command-Option-O-F), and type :

setenv boot-device hd:2,yaboot (where 2 is the number of the partition

containing yaboot)

boot



And here you go, you can unplug that useless monitor 



References :

http://saintaardvarkthecarpeted.com/blog/archive/2005/02/THE_WINE_OF_BOOT.html

http://grub.enbug.org/TestingOnPowerPC



By the way, as of writing this, grub2 on PPC from debian gives a "out

of memory" issue. Known bug.

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