I'm waiting on some things in the mail, and then I'll see what I can put together.
The ESP8266 only has one fully-functioning UART, and it's connected by default to the USB programmer. An additional problem that I had not considered is that the ESP8266 spews out some informational messages upon booting, before any uploaded firmware is even run. It looks like the safest bet is to put the SIO DATA IN/DATA OUT on GPIO15 and GPIO13, respectively. (See http://smallbits.marshall-tribe.net/blog/2016/11/13/esp8266-quiet-uart) That way, the startup messages will go to an otherwise unused pin, and if you want to divert the UART to these other pins, you can. Or you can use some software-based serial scheme on these pins. The advantage to diverting the UART is that the serial communication is handled by hardware; the disadvantage is that you won't be able to send messages out the USB port as you attempt to debug it.
The remaining SIO signals can go to other various pins. The 850 almost certainly uses the COMMAND signal, but probably not the CLOCK ones. The others I'm not sure about. Since SIO uses 5V signals and the ESP8266 is a 3.3V device, I plan on using two 74AHCT125s as level shifters, one for incoming signals and one for outgoing ones. Each can handle four signals. These would probably be in: READY, COMMAND, DATA OUT; out: DATA IN, PROCEED, INTERRUPT. Let me know if you think more connections are needed.
Also, I did get a cyber1 account and can access it using pterm, but I currently lack the means to load the Atari Plato software. I have a MaxFlash setup on the way.
Edited by spiffster, Wed Mar 14, 2018 3:13 PM.