I don't use ZeroTeir but would first ask: how are you applying your customization? Is it direct commands/script based? If so you could just drop them into /etc/rc.local so they run at boot. Don't forget to add that file to /etc/sysupgrade.conf or you'll lose them if you upgrade the firmware. That's the quick & dirty method.
I follow the more traditional UNIX approach. I put all my shell scripts in /root/.bin & update the $PATH to point to it. See below for specifics. That path/a script within can the be called anywhere inc. rc.local. Don't forget to add /root to /etc/sysupgrade.conf for the same reason above.