DaemonNews.com has the how-to: "Previous work to make a read-only mounting of root and /usr on a normal hard disk was working for Sparc and i386 in October
2000. With upgrades to -current every now and then, I noticed that with my changes to tcpdump, /etc files, cg3 and cg6 drivers,
dhclient-script, miscellaneous others -- let alone the */conf/ files to hard-wire device numbers to disk and tape drives, that it was
nearly impossible to keep up with the development changes."
June 6, 2001