There’s “custom” Firmware for the WDTV Live Streaming
http://forum.wdlxtv.com/viewtopic.php?f=66&t=8707&start=0
and also for the WDTV Personal Edition
http://forum.wdlxtv.com/viewtopic.php?f=23&t=8974&start=0
But both are based on “Official Firmware” …
(i don’t believe it’s possible to build a Firmware from “Scratch” as i’m pretty sure there’s WD Proprietary code that can’t be altered)