Tasha Upchurch

If data need to be recorded network boot with nfs will be needed but if it's just to run an small emmbedded project, it's possible to load rootfs in ramdisk and remove the sdcard in case you don't use a spi flash to boot.
It's work fine for sure with openwrt but armbian is too big to fit in ram

Haven't seen Wifi support but its probably doable (probably undeveloped, which means some hard work has to be done) ... In general, wired network support is at best and also I am not sure if this is enabled on all devices. This feature is not that much important to our case.
 

This should also work if you boot from the network. 

I saw this: https://linux-sunxi.org/FEL/USBBoot#General_description_of_the_.22sunxi-fel_uboot.22_command_implementation
 
And google this: OrangePi Zero FEL  - Result: There are no buttons or connectors to select FEL mode so the BootROM will only enter FEL mode if a special SD card is present or if there are no valid boot options.
 
And a forum search: https://forum.armbian.com/topic/2808-orange-pi-zero-went-to-the-market/?do=findComment&comment=20334
 
But by all means, I have never tried it, but I would like to do it. TK quite easily tested new Kernels that way, IIRC.

Overlay file system is NOT what you want, it requires the original media to be readable.   I'd recommend a u-boot that handles net booting (if it exists) loaded to the on-board flash, then a network root and swap if needed.  One system would require real media and the others a network connection to it.

Well, I don't know if the OPi zero can do FEL https://linux-sunxi.org/FEL/USBBoot

Hi, no experience with this here, but did you read:
 
The note about overlayroot?
 
best, gnasch