Here is the original RockBox sbloader.c that I used when I started my port:
http://git.rockbox.org/?p=rockbox.gi...ols/sbloader.c
I have made a lot of (debugging) changes to my copy of it, including changing the USB VID/PID to what K4 and K5 USB Downloader mode uses.
EDIT: I fixed the URL. My VM has occasional copy/paste problems.