Hi,
do you know, is there any thing like Linux From Scratch, but for a BSD operating system?
A tutorial or guide to learn (and build) the system inside out? Would be pretty cool IMO. 
Blackbird
There is no LFS for FreeBSD, but FreeSBIE (FreeSBIE - Free System Burned In Economy) was one of the first to try it and then DesktopBSD (DesktopBSD: Home) came along. Both projects have been abandoned, but you are welcome to look under the hood to modernise the install scripts.
As an alternative, you may want to start with the installation of a bare FreeBSD system using sysinstall and build your post-installation scripts from there with the appropriate interactive compiler options and libraries.