I asked this question a couple of years ago but didn't pursue it.  I 
now have a more urgent need for an answer:  how do I build a bootable, 
*non-install* NetBSD CD for i386?

Specifically, I want to put a file system on a CD, plus a boot block 
that will load the kernel from that CD, and run it, multi-user.  I want 
X, I want /usr/pkg, I want amd -- I want a running NetBSD system that I 
can use on a machine that has nothing but Windows on its hard drives.

The instructions at http://www.netbsd.org/Documentation/bootcd.html
say to use boot-big, but that always dumps me into sysinst, which isn't 
what I want.  When I asked a couple of years ago, someone suggested 
putting 'root on cd0a' in my config file; I did that, but it didn't do 
what I wanted.

There are other interesting challenges, such as what to do about applications
that absolutely demand a writable file system.  I may play around with
mount_union (or mount_null or mount_overlay, as soon as I understand
the differences between the three....) to overlay / (or at least /usr)
with an mfs file system very early in the boot process.  But I need
to get CD booting working first.

I tried mkisofs with biosboot.sym, but it complained that it was the
wrong size.  Could I use vnd to mount a .iso file and then use
installboot on it?

I'm sure that someone has done this before; it's a fairly obvious thing
to want to do.  But I haven't figured out how to make it work.