I splitted (and modified) the previous busybox patch.
The patches not sent upstream are [0] and [1].
To build busybox on Debian GNU/kFreeBSD you need:
-applying the 2 patches linked in this post
-applying the 2 patches sent upstream
-copying the "udeb" config also to "deb" and "static" ones
[0] http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/20090515/20090514_2011_busybox.debian.config.diff
[1] http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/20090515/20090515_1739_busybox.debian.source.diff