Hmm, not sure since your old root partition wouldn't mount but this may be an option: http://wiki.sabayonlinux.org/index.php? ... ecover_GCC
That is for GCC (gnu C compiler), but just change the command from gcc to bash and it should work.
Like I said, requires the ability to actually mount your old root partition, which from your post seems like it didn't work. This also leads me to believe that you did other nastiness to your install other than just uninstalling bash. /dev/sda1 (or whatever root was) should still mount on the livedvd even with bash gone.
It may be that even the upgrade won't work because the livedvd needs to mount your harddrive partitions to install/upgrade packages.
EDIT: Forgot to tell you something sorry