Ok, I just checked the installation of my drive and got it to boot on the macbook with the additional kext from the AppleTV.
I think it is very much the same as booting it on AppleTV.

Here is how to setup the drive for the AppleTV:

installing:
- remove the drive from the system
- set it up to a system with a install of OSX
- install OSX to the drive, (the diskutil will setup your EFI boot partition, don't worry on this it's common for OSX)
- boot it up and set it up
- reboot to the other OSX

Setting up the system:
- remove mach_kernel from / of the volume containg the isntall for ATV
- put my special mach_kernel into the / of the ATV drive
- replace /System/Libary/Coreservices/loginwindow.app with the one from ATV
- add the additional kext from ATV /system/libary/extensions/ to the target system
- remove AppleWatchdog* kext
- delete /system/libary/extensions.mkext and extensions.kextchache (IMPORTANT!)
- do diskutil repairPermissions /Volumes/yourATVdrive from the other OSX

If kernel is working, this MUST boot. In case you face further problems, If you change kext things, be sure to delete the mkext and kextcache EVERY time.

If it still stays with "?" try rename drive to OSBoot like the original ATV.

If something goes wrong send me /var/logsystem.log if available. And add -v to your com.apple.Boot.plist in the propper place toenable verbose mode. Maybe we should try -s as well and boot ATV into single user mode lol.

In case of problems, post your query for anyone here in this forum, I will answer it and anyone hitting the same "spot" can have the chance to look it up.

Originally Posted by semthex

- set it up to a system with a install of OSX
- install OSX to the drive, (the diskutil will setup your EFI boot partition, don't worry on this it's common for OSX)
- boot it up and set it up

I used MacBook Pro 10.4.6 install DVD. Is it important to update the clean build to 10.4.9 or any of the other updaters? Or can I just run this 10.4.6 with your changes?

For anyone to read:

kernel will onyl run 10.4.8 surounding ++