Did you document your process? I'd love to read about it
Did not document (other than the assembly script that I can't share), although I did post my jail.conf (essentially) in an earlier FreeBSD thread https://news.ycombinator.com/item?id=10602990
If you know how to build a minimal chroot, you know how to build a minimal jail, and then you just need to configure it, etc... If you don't know how to build a minimal chroot, check out option 3 'most minimal chroot possible' here: http://ram.kossboss.com/debian-ubuntu-build-minimal-chroot/
It's written against Debian Linux, but the same thing basically works for FreeBSD, except ldd on FreeBSD omits the loader (/libexec/ld-elf.so.1), but I sort of remembered that from somewhere, and I used truss to see what system calls were being used before it died.
It's not as obvious or well documented how to set it up this way, but it was maybe a couple days work, including my first rc script ever, and the script to assemble the jail in our environment (you could probably have a tar file with the contents of the jail, and just add the conf/data files)