In your example:

  REAL_PACKAGES="/usr/pkgsrc/packages"
  REAL_PACKAGES_OPTS="-t null -o ro"

I think this should be rw; processes in the chroot have to write to
this directory.

  ===> Checking for vulnerabilities in pkg_install-20050318
  [...]
  pkg_admin: Cannot chdir to /var/db/pkg: No such file or directory

I would expect unpacking the sets to have created /var/db/pkg.  But if
building newer pkg tools succeeds and this gets created in the
process, your chroot is probably usable.

-- 
        Greg Troxel <gdt@ir.bbn.com>