I managed to build busybox on GNU/kFreeBSD with "CONFIG_INIT=y".
I couldn't test the binary because I don't know how to test it...
Here is the patch:
http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/config_init/20090521_1958_busybox.config_init.diff
(old version of the patch)