Laurent, I didn't have time to take the variant with flow control further than breadboard level.
However, adding flow control doesn't bring the huge benefit one might imagine. One thing the ESP8266 WiFi microcontroller has very little of is RAM. We're talking about some kilobytes(!) available to the user. When the Atari -- using RTS/CTS flow control -- tells the module to stop sending but packets still arrive from the network, they have to be buffered inside the ESP8266. As the memory rapidly overflows, packets will be lost either way. Hence, I found it's better to configure the Atari in such a way that it doesn't have to use flow control most of the time.
Nevertheless, I might draw the schematic and post the firmware for the version with flow control the coming weekend. As for PCBs, there hasn't been much interest right now, so I didn't feel the necessity to design some.