Download: download.zerotier.com/RELEASES/1.4.6/dist/ubiquiti
Place a copy of the package in: /config/data/firstboot/install-packages/
Install: sudo dpkg -i /config/data/firstboot/install-packages/zerotier-one_1.4.6_ubiquiti_mips.deb
Your local node configuration will be stored in /config/zerotier-one which persists across firmware upgrades. When you upgrade firmware the package will be automatically reinstalled (from /config/data/firstboot/install-packages/), started, and will use the config left over in /config/zerotier-one.