It has been a while since I have done these sort of things, there are possibly faster/easier ways. Going off what I knew 3 years ago and the fact you have a custom bios, i would suggest...
Slap in slayers and FTP to the box, you can rename anything you know is wrong, personally if an xbox was sent to me doing that, I would wipe the c drive and download the latest build of XBMC and overwrite anything that was there.
Other way, would be of course as you stated, the drive is unlocked so simply open her up and plug her into the computer. Be sure to have a utility able to read the fatx partitions. Once again, if it were me, i would download xboxhdm and the latest xbmc, use xbrowser to clean out the C drive and transfer over the new dashboard.