I need also to find a way to connect this Pi zero...so I have a linux on this side...alone in the world
Maybe program a tun/tap program on PI to send receive data (uart/spi) by ESP8266 connected as relay to a socket server on an other linux router AP (openwrt), with also support tun tap standard? I see only this way for the moment...not very flexible but ESP can be configured by SPI, UART to configure Server:port
I looked for by forwarding interface on ESP AP<-->STA - No way for the moment
With ethernet converter - No way for the moment with ESP, but OK for PI.