A more permanent change to readonly squashfs mounted /etc

I have an IP-camera with the BusyBox v1.22.1 operating system that has some accounts either without or with public known passwords that I want to change to make it more secure. I have managed to log in to the root account via telnet but could initially not change password or users. The filesystem is squashfs and except the /tmp and /home directories all other direcories are mounted as read only. After some searches on the internet I found a way to change the contents of files in the /etc directory where the password and shadow files are stored. I used bind-mounting of a copy of the /etc directory in the /home directory as the /tmp directory was nor persistent during reboot. /tmp is probably just stored in ram memory. I used the following command: cp -a /etc /home/tst && mount --bind /home/tst/etc /etc but the bind-mount is not persistent during reboot so I need to find a file that is exceuted in the boot process that does the binding (eg where I can put the "mount --bind /home/tst/etc /etc" command) as the files in the /home directory is not affected by reboot. What file would be good to have the mount --bind command in, preferably a file in the /home directory that is executed in the last part of the boot process so it is not overridden by some other command?