Yep, I had the same problem. Only MorphOS & OS X dualboot guides. So I had to try it out on my own. But as I said, it's not that hard if you already have a working yaboot.conf.

Which OS is installed first does not matter in the end. The yaboot.conf must match your real partition layout and you need to finally write your with the ybin command as I said.

If you make a screenshot of your partition layout, e.g. with gparted I can guide you through.

The 'magic' 5th partition is always sda1, a small 32 KiB partition at the beginning of the disk. This is where the Mac partition layout (and the boot sector?) itself is stored. I didn't know that either before. The organisation of the Mac partition layout seems different than MBR or GPT which is used on PCs or RDB which is used on Amigas.