It is sometimes possible force the rc.s scripts to not run by appending the init directive to your exec command in Redboot:
exec -c "console=ttyAM0,115200 root=/dev/mtdblock1 init=/bin/sh"
The way you would do this is to use ctrl-c during the boot process to reach the Redboot prompt. Once that is done, you can run the commands to start Linux by hand:
fis load vmlinux
exec -c "console=ttyAM0,115200 root=/dev/mtdblock1 init=/bin/sh"
This will cause the system to jump immediately to the shell prompt after executing the kernel, skipping the normal boot files. From there you can make your modifications to fix the erroneous rc.S boot system.