The best method to disable any sort of screen blanking is to turn off DPMS and set the blanking delay to 0 in xorg.conf.
I simply inserted these two lines in /etc/rc.local:
su linaro -c "xset -display :0 -dpms" su linaro -c "xset -display :0 s off"
This has the disadvantage that should XBMC crash, and it does happen now and then, then it will reload the default settings. I have not found this to be a problem.