I got Pi Zero USB OTG working with OpenWrt 19.07.4
The hang up I found was getting the g_ether module to load which creates the 'usb0' interface. On the normal Raspi OS I believe it's triggered in the cmdline.txt in the boot partition, but that didn't work in openwrt so I had to load it manually after boot.
'modprobe g_ether' should load it, and
'modinfo g_ether' should confirm it loaded.
After that you can create a interface that uses 'usb0'
Later added a link in modules.d so it loads each boot.
'echo g_ether > /etc/modules.d/56-g_ether'
For reference I partially used this guide to get the Pi Zero initially setup.