As you know Busybox is a base program responsible for almost all commands used in PlayOn. The bad thing is that every single PlayOn!HD firmware is based on version 1.1.3, which was made on 27 December 2006 (so it's quite old).
I've compiled Busybox 1.18.1 (released on 21 December 2010) for POHD: Download
Documentation: HTML | TXT
There's also standalone Busybox 1.18.3 (symlinks are the same as above).
Update: Busybox 1.20.2 (documentation available in: HTML | MAN | TXT)