I managed to get free in busybox working on GNU/kFreeBSD, but heavily copying and pasting code from the procps package.
The resulting free implementation is not so tiny...
I quickly tested it (busybox free), and its output looks ok.
Get the patch at:
http://slackydeb.altervista.org/files/debian_gnu_kfreebsd/busybox/free/20090519_2342_busybox.diff