結局、こうやってみた。

%df -t nullfs
Filesystem 1K-blocks Used Avail Capacity Mounted on
/md/usr/local/bin 396526 103588 261216 28% /usr/local/bin
/md/usr/local/sbin 396526 103588 261216 28% /usr/local/sbin
/md/usr/local/lib 396526 103588 261216 28% /usr/local/lib
/md/usr/local/libexec 396526 103588 261216 28% /usr/local/libexec

(立ち上げ時に実行)
# XXX memory disk hack
rm -rf /md/usr
( cd / ; tar cf - usr/local/bin ) | ( cd /md ; tar xBpf - )
( cd / ; tar cf - usr/local/sbin ) | ( cd /md ; tar xBpf - )
( cd / ; tar cf - usr/local/lib ) | ( cd /md ; tar xBpf - )
( cd / ; tar cf - usr/local/libexec ) | ( cd /md ; tar xBpf - )
mount_nullfs /md/usr/local/bin /usr/local/bin
mount_nullfs /md/usr/local/sbin /usr/local/sbin
mount_nullfs /md/usr/local/lib /usr/local/lib
mount_nullfs /md/usr/local/libexec /usr/local/libexec