it wouldn’t be hard. Its just outside of the scope of this project.
How you would do it would be (this is how I would do it) You could also just add an entry into the ports section, that’d be easier.
Create entry in es_systems.cfg for XBMC
1.0 location: ~/.emulationstation
2.0 location: /etc/emulationstation/
Copy the AppleII theme into /etc/emulationstation/themes/simple/xbmc
make a new xbmc_art_blur.png, xbmc_art.jpg and xbmc_logo.svg
I just modify the AppleII ones, and it plugs in pretty well.
then change the commandline to be like <command>xbmc</command>