In theory it's possible: first, you should modify /etc/X11/xorg.conf, I think you need to modify things in the "Device" section (well, Xorg nowdays works fine without xorg.conf, as you noted too, so the other solution is rename xorg.conf so X won't use it).
But it's only the half part of the story, the other part is the kernel driver. I would "blacklist" fglrx as a kernel module, so it won't be load, when you don't need it: /etc/modprobe.d/blacklist.conf
I don't have a setup for ATI card in this way, but I have similar for nvidia, to switch between nvidia's binary driver and nouveau (open source driver for nvidia cards).
Best Answer
To start Steam without fglrx, type this in Terminal:
Terminal has to be active, for small games (Stellaris by my own) it's practicable.