Tristan> "HFS or HFS+ file system (Open Firmware 2.4, Open Firmware 3) 

Tristan> Whilst you can load ofwboot.xcf from an HFS or HFS+ partition, you will
Tristan> not be able to load the kernel from HFS or HFS+ filesystems. 
Tristan> resolved in a future release of NetBSD/macppc"

(may be kind of resolved.)
In this context, load the kernel means normal /netbsd after installation.
You can not mount and use the kernel residing HFS or HFS+. It should be
placed on 4.2BSD (A/UX root or /usr etc.) partition.

Tristan> then, a few bits downstream, I read this:

Tristan> "Boot off an HFS or HFS+ partition (Open Firmware 3) 

Tristan> Expander to uncompress the kernel. Get to the Open Firmware prompt. Boot
Tristan> the kernel directly (i.e. without the use of a bootloader) and use it to
Tristan> install NetBSD on your hard drive."

The boot loader, ofwboot.elf ( or ofwboot.xcf), placed on the HFS or HFS+
partition may load the installer kernel from HFS or HFS+ in this case.

The correct expression for this issue is:

  Boot loader (ofwboot.elf or ofwboot.xcf) may read the kernel either
on 4.2BSD or on HFS/HFS+ partition, but the kernel on HFS/HFS+ may not
be mounted after the kernel starts. That is why usual running kernel
has be reside on 4.2BSD partition side.

And, you don't need to uncompress the kernel anyway.
just type from the Open Firmware:

0 > boot hd:,ofwboot.elf netbsd.GENERIC_MD.gz

(In this case netbsd.GENERIC_MD.gz is expected to be on the same partition
as ofwboot.elf )
---
Makoto Fujiwara, Chiba Japan
Manual installation instruction for New Macintosh:
http://www.ki.nu/software/NetBSD/iBook2/