ok, so the bug isn't in the runtime configuration (live-config) but in fglrx doesn't work with kms which we'll need to workaround in the buildtime configuration (live-images) by conditionally adding nomodeset to bootappend in case non-free was enabled.
I don't use fglrx - my graphic chips (Radeon Xpress 1250 (200m) and Radeon Xpress 1150 (200m)) work only with radeon driver and R300 firmware. What has fglrx and kms for my case? Yes, it start with "radeon.modeset=0" parameter, but in this case it start with software rendering. Live images from other ditributions (openSUSE, Fedora) that have nonfree firmware start perfectly with hardware rendering for radeon driver and with kms.
Maybe radeon driver should be included in initrd image?