Did you try asking people at www.libretro.com ? They have a port of retroarch to almost any platform including xbox360 so perhaps they could help.
@kev : Do you mean fbanext ? (found the code there : https://github.com/magicseb/fbanext)But i'm not sure he is asking specifically about fba here.
Lantus and I started a 360 port ages ago and I think he did even finish it to some degree. I don't seem to be able to find a copy of the code anywhere though. Either way, you might get ahold of him on here you via youtube https://www.youtube.com/channel/UCjFaPUcJU1vwk193mnW_w1w