I am by no means a NetBSD expert, and I'd be happy to hear from anyone who can provide me with better work-arounds than these.  However, here is how I got a working version of openplans up and running on NetBSD (version unknown, its one of our development servers.)

1) If you get an error message like:
tar: Failed open to read on /dev/nrst0 <No such file or directory>

Then you will need to change /usr/pkg/lib/python/site-packages/topp.build.lib<somestring>.egg/topp/build/lib/commands.py so that the line:

cmdline = ['tar', 'xv']

reads:

cmdline = ['gtar', 'xv']

2) You need to use gmake instead of of make, edit the  /usr/pkg/lib/python/site-packages/topp.build.opencore<somestring>.egg/topp/build/opencore<somestring>.egg/topp/build/opencore/tasks/zope.py file so that this line:

compile_zope = Task(
    'Compile Zope',
    namespaces='zope',
    targets=['${zope_source}/bin',],
    workdir='${zope_source}',
    commands=['./configure --prefix=${zope_home}',
              'make',
              'make inplace',
              ],
    dependencies=(patch_zope,)
    )

reads:
compile_zope = Task(
    'Compile Zope',
    namespaces='zope',
    targets=['${zope_source}/bin',],
    workdir='${zope_source}',
    commands=['./configure --prefix=${zope_home}',
              'gmake',
              'gmake inplace',
              ],
    dependencies=(patch_zope,)
    )

3) The accept certificates script may not be executable.  Run the following command:
sudo chmod +x /usr/pkg/lib/python2.4/site-packages/topp.build.opencore<somestring>.egg/topp/build/opencore/skel/bin/accept_certificates.sh

If that does not fix the problem, then bash may not be installed.  Change the first line of the file from:

#!/bin/bash

to

#!/bin/sh

[Of course, you could do the smart thing and just install bash -- myddrin]

3) Make sure that you have the libxml2 and libxslt1 libaries and headers installed.

4) You may have problems with the zope make process complaining that pyexpat is not installed.  I resolved this by copying /usr/pkg/lib/python/site-packages/_xmlplus and /usr/pkg/site-packages/pyexpat.so to ${deploy_dir}/lib/python.  This worked for me but is not optimum.

[I'm not sure what the issue is with this one, but this seems like a reasonable work around for now.]

5) nmap is not installed by default, you will need to install it.

6) In the zopectl file, change #!/bin/bash to #!/bin/sh

Item 5 should be fixed with the latest version of topp.build.opencore.

Patches for items 1 & 2 have been submitted.