I've played a bit with the MacOS X install CD to see what it does to the 
disk during installation, and this might offer another clue for making 
NetBSD bootable volumes.

I dumped the Disk Partition Map on the target disk before and after MacOS 
X installation.

Two new Partition Map Entries show up after installation which were 
carved out of the space I'd allocated for the MacOS X UFS partition:

  pmPartName: MOSX_OF3_Booter
  pmParType:  Apple_Boot
  pmPartBlkCnt: 0x4000
  pmPartStatus: 0x40000077
     none of the Boot parameter fields are filled in and I don't know if 
anything was loaded into the partition during installation.

and:

  pmPartName: SecondaryLoader
  pmParType: Apple_Loader
  pmPartBlkCnt: 0x400
  pmPartStatus: 0x4000007f
  pmBootSize: 0xad000
  pmBootAddr: 0x1c00000
  pmBootEntry: 0x1c00a70
  pmProcessor: powerpc

I'm no expert, but it seems to me that Apple might be creating a new 
Apple_Boot partition that is meant to contain the OF3_Booter code if it's 
not already on the machine.  Could this be a way of supporting systems 
that don't have OF3 onboard?  The second new partition appears to contain 
the Secondary Loader independent of where the OS is in the Apple_UFS 
partition.

-bob

p.s. Other MacOS X reports indicate that after installation some HFS/HFS+ 
disks appear to be missing. It appears that maybe the install process is 
only allowing one MacOS 9 bootable HFS/HFS+ to remain visible.  I had two 
(one HFS and one HFS+) and only one (the HFS+) remained after 
installation, although it could still be selected from the Startup 
Control Panel under MacOS 9 and the system would boot to it if it were 
selected. The only change I can find in the Partition Map Entry for the 
visible HFS+ partition is the pmPartStatus; it changed from 0xc400037f to 
0xc000037f.  The "hidden" HFS partition status remained unchanged with 
0x40000037f.