Can you also try blacklisting radeon and loading it after the box has booted
and you have remote access?  E.g.,  append modprobe.blacklist=radeon 3 to the
kernel command line in grub (the 3 is to boot into runlevel 3 so X doesn't
start).  Then once the system is booted and you have remote access, as root,
run: modprobe radeon
and see if you can get the dmesg output.