You can put a sleep command before poweroff.
sleep 300; poweroff
This way you will have 5 minutes before the poweroff. When you want to stop this, you can use these 5 minutes to login to the system and remove the poweroff command from the init script.
Another way would be to add
init=/bin/sh to the kernel parameters upon booting (in the bootloader), when you want to stop this. Then only the kernel will be booted and no init scripts will be ran. You will endup in root shell. Since no scripts were ran you will need to remount your RootFS with RW, like this:
mount -o remount,rw / then remove the init script,
sync the FS and
I would use the first option, it's more safer if you don't know what to do.