First, I bootstrapped pkgsrc into the directory /usr/pkg/2006Q4.
$HOME/proj/pkgsrc/bootstrap/bootstrap --prefix=/usr/pkg/2006Q4 --unprivileged --compiler=sunpro
mkdir /usr/pkg/2006Q4/etc
mv work/mk.conf.example /usr/pkg/2006Q4/etc/mk.conf
Then I made a backup of that directory. You can never know. 
cd /usr/pkg && gtar cfz 2006Q4.tar.gz 2006Q4
Since in a previous try, the pre-build program had removed the pkgsrc directory, I just commented it out by inserting an exit 0 in line 2. Since now there is no danger that anything in that directory is deleted, I could edit the mk.conf file and adjust some local settings.
/usr/pkg/2006Q4/mk.conf
# Example /usr/pkg/2006Q4/etc/mk.conf file produced by bootstrap-pkgsrc
# Tue Jan 9 13:01:49 CET 2007
.ifdef BSD_PKG_MK # begin pkgsrc settings
PKGSRC_COMPILER= sunpro
UNPRIVILEGED= yes
PKG_DBDIR= /usr/pkg/2006Q4/var/db/pkg
LOCALBASE= /usr/pkg/2006Q4
VARBASE= /usr/pkg/2006Q4/var
PKG_TOOLS_BIN= /usr/pkg/2006Q4/sbin
PKGMANDIR= man
TOOLS_PLATFORM.awk?= /usr/pkg/2006Q4/bin/nawk
TOOLS_PLATFORM.sed?= /usr/pkg/2006Q4/bin/nbsed
FETCH_CMD= /usr/pkg/2006Q4/bin/ftp
TOOLS_PLATFORM.pax?= /usr/pkg/2006Q4/bin/pax
TOOLS_PLATFORM.tar?= /usr/pkg/2006Q4/bin/tar
TOOLS_PLATFORM.mtree?= /usr/pkg/2006Q4/sbin/mtree
DISTDIR= /usr/pkg/distfiles
PACKAGES= /usr/pkg/2006Q4-packages
PKG_DEVELOPER= yes
CC= cc
CXX= CC
CPP= cc -E
CXXCPP= CC -E
SUNWSPROBASE= /local/SUNWspro
USE_LANGUAGES+= c99
TOOLS_PLATFORM.nroff= /opt/bin/groff
.endif # end pkgsrc settings
/usr/pkg/2006Q4/build.conf
# $NetBSD: how_to_do_an_unprivileged_bulk_build_on_solaris.mdwn,v 1.2 2012/02/05 07:14:36 schmonz Exp $
#
osrev=`uname -r`
arch=`uname -m`
USR_PKGSRC="$HOME/proj/pkgsrc"
MAKECONF="/usr/pkg/2006Q4/etc/mk.conf"
PRUNEDISTFILES=no
NICE_LEVEL="nice -n 20"
LINTPKGSRC_CACHE=no
ADMIN="1illig@informatik.uni-hamburg.de"
ADMINSIG="- Roland"
REPORTS_DIR="/usr/pkg/2006Q4-pkgstat"
REPORT_BASEDIR=`date +%Y%m%d.%H%M`
REPORT_HTML_FILE="report.html"
REPORT_TXT_FILE="report.txt"
REPORTS_URL="file://$REPORTS_DIR"
UPDATE_VULNERABILITY_LIST=yes
PRUNEPACKAGES=yes
MKSUMS=yes
MKSUMMARY=no
RSYNC_DST=ftp.NetBSD.org:/pub/NetBSD/packages/pkgsrc-200xQy/NetBSD-a.b.c/i386
RSYNC_OPTS='-e ssh'
I began to setup the bulk build environment in a screen.
$ screen -S <ulk-2006Q4
$ sh ./proj/pkgsrc/mk/bulk/build -c /usr/pkg/2006Q4/etc/build.conf
It seems to work, and I'm waiting for the package databases to be generated. But since I have a T2000 here, I'm thinking about parallelizing the work. For generating the databases, the two "make" processes could be run completely in parallel, making use of 12000 processors.
In the main phase, all packages that have the exact same dependencies can be built in parallel, but must be still installed one after another.
Help, my files are being removed
When I had started the bulk build, it complained that it couldn't find /usr/pkg/2006Q4/bin/nawk anymore. But looking at the backup I made above, I say that it had been there after bootstrapping. The simple solution to this was to change the tools definitions in mk.conf so that they point to tools outside ${LOCALBASE}:
TOOLS_PLATFORM.awk= /usr/pkg/current/bin/nawk
TOOLS_PLATFORM.sed?= /usr/pkg/current/bin/nbsed
FETCH_CMD= /usr/pkg/current/bin/ftp
TOOLS_PLATFORM.pax?= /usr/pkg/current/bin/pax
TOOLS_PLATFORM.tar?= /usr/pkg/current/bin/tar
TOOLS_PLATFORM.mtree?= /usr/pkg/current/sbin/mtree
Luckily, bin/bmake is not registered as belonging to any package, so it hasn't been removed yet.
The bulk build also tried to deinstall the infrastructure packages, so I had to protect them against that:
BULK_PREREQ+= pkgtools/bootstrap-mk-files
BULK_PREREQ+= pkgtools/tnftp
BULK_PREREQ+= pkgtools/mtree
BULK_PREREQ+= pkgtools/pax
BULK_PREREQ+= pkgtools/pkg_install
BULK_PREREQ+= sysutils/mtree
BULK_PREREQ+= sysutils/checkperms