Success!  My iMac is converted to using Apple Partition Map
and will boot directly from the hard drive.  A quick summary:

+ install from CD, use a NetBSD disklabel and leave free space
  at the front of the drive (~10MB for ofwboot.xcf and some
  kernels)
+ boot from CD
+ install sysutils/hfsutils package
+ run the output of disklabel(8) through a Python program to
  generate an APM image
+ dd(1) the APM image onto the disk
+ hformat(1) the newly created HFS partition
+ use hcopy(1) to put ofwboot.xcf and netbsd kernel on the
  HFS partition
+ reboot from HD :-)

In theory, anyways... it works with a NetBSD-1.5.2 system and
netbooting.  I'm going to start from scratch and install 1.6,
document the whole process, and convert the Python program to C.
Stay tuned, results should be posted in a few days.

Thanks again to Bill Studenmund, who has been especially helpful
in answering several rounds of questions, saving me many days
(weeks?) of trial and error experimentation.

-Neil