busybox "mount" builds (I think it does not work...) on GNU/kFreeBSD with a very hackish patch [0] (Update 20090603: obsolete).
Linux "mount" has one more explicit parameter (i.e. "source") than the kFreeBSD one, as you can read from their man pages ([1] and [2]).
I should try to use nmount on kFreeBSD, using "mount" in freebsd-utils as a reference.
[0] http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/mount/20090602_2218_busybox.mount.diff
[1] http://www.freebsd.org/cgi/man.cgi?query=mount&apropos=0&sektion=2&manpath=FreeBSD+7.2-RELEASE&format=html
[2] http://www.kernel.org/doc/man-pages/online/pages/man2/mount.2.html
Update 20090603:
I updated the patch:
-using nmount as in freebsd-utils
-moving platform specific code into platform.h
Now I have to test busybox "mount" with this patch.
Get the patch from
http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/mount/20090603_0028_busybox.mount.diff
Update 20090604:
I updated the patch to export a useful function outside the file to "help" umount.
I enabled "umount" and made it building.
I tested busybox "mount" and "umount" with these patches (with a msdosfs filesystem).
Get the patches from
http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/mount/20090604_0044_busybox.mount.hack_to_help_umout.diff
http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/mount/20090604_0045_busybox.umount.diff