QNAP helpfully explains how to add items to crontab on one of the NAS machines on their wiki here. Less helpfully they don’t explain that adding jobs with crontab -e and then restarting the cron daemon won’t persist through a reboot. Yay. However, you get around this by adding jobs directly to /etc/config/crontab. Once you’ve done that reload the crontab file and restart the CRON daemon with:

crontab /etc/config/crontab && /etc/init.d/crond.sh restart

Thanks fellas.