I ended up creating nine partitions to support three OSes, three of which weren't strictly necessary. But, with mac style partitioning allowing up to 16 partitions, I would still be able to fit in those seven partitions described for FreeBSD.

After three failed attempts (one each for those three OSes) with each OS's partitioning tool, I decided to start over with OS X as it seemed unable to install if the drive had been partitioned by any other OS. Installing OS X first was no problem, Ubuntu seemed to have a mild issue allowing me to mark a partition as Newworld bootstrap and as /boot, but when I continued with the install, it seemed to figure out what to do.

Then MorphOS... From previous attempts I'd noticed that if iWizard was going to have a problem there'd be some weird installer window resizing behaviour at the beginning of the install process. It's difficult to be sure but partitioning with OS X seems to cause the MorphOS installer partitioning tool to show double entries for each partition - even those nutty 128MB padding spaces. Clicking on any partitions results a 'bubble' message:

Invalid partition layout!
Valid sector range is between
Sector x and Sector x-1.
Maximum size is 0.
Be aware that the Apple Partition Map
at the start of the drive requires
64 cylinder(s).

Not knowing how to resolve that error, I quit the installer and went about trying to do a manual install. HDConfig displayed the partitions as expected so I chose the one I wanted to use for MorphOS, changed it to SFS and set it as bootable. I also edited the partition I wanted to use as MorphOS Boot. Then I simply copied the contents of the MorphOS install cd to the SFS partition and copied boot.img to Boot.

I followed the instructions from the beginning of this thread to be able to boot MorphOS.

Q Does having a bootloader like yaboot remove the need for other OSes to have individual boot partitions with their single, blessed boot files?

I know this would prevent the Apple alt-boot menu from displaying the available OSes but, as long as yaboot can provide a menu...

After installing MorphOS, OS X shows a requester advising that Boot should be repaired. I get the impression that OS X still thinks Boot is a HFS Extended partition even though I reformatted it in MorphOS as a plain HFS. I also created a FAT partition thinking each of the OSes would be able to mount it and that I could use it for sharing large files between them. But OS X won't mount it. I guess what might help is a decent replacement for Disk Utility...