Thanks Daniel! I'm looking into it, I found the xserver-xorg script in /lib/live/config... and it's exactly what I was talking about.
So, if I'm getting this right, thanks to the xserver-xorg script, any custom image with the non-free nvidia/fglrx driver packages installed should automaticaly pick them up, configure them and choose it with update-alternatives, right? And the script is automaticaly run at boot time, like all the scripts from live-config, right?
If that's so, I'm having problems in (at least one) PC with an AMD graphics card that in update-alternatives has glx selected as mesa-diverted... which shouldn't be happening.