Busybox shouldn't be Linux-specific [0].
At the moment it doesn't build on GNU/kFreeBSD [1].
There were previous porting efforts [2][3].
I'm trying to make it building [4].
I am only considering the "udeb" config (busybox-1.13.3/debian/config/udeb).
At the moment I am having problems on libbb, and it seems to me that disabling more applets is unuseful to skip this error.
Here [5] is the build log.
[0] http://www.busybox.net/downloads/README
[1] http://buildd.debian-ports.org/status/package.php?p=busybox
[2] http://glibc-bsd.alioth.debian.org/patches/
[3] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=323670
[4] http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/20090508_1659_busybox.diff
[5] http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/20090508_1734_busybox_build_log.txt