What about new version of module ESP8266WiFi

Just a quick note - if you're not running anything on the Pico then A2/A3 will be totally open circuit. You could plug USB-TTL directly to the ESP8266 and program it that way.

I just checked, and on the latest Espruino Images (the USB HID ones - I haven't checked others), you can actually do the following:

• Connect to Espruino
  

• Copy and paste this code into the left-hand side:

  Serial2.setup(9600, { rx: A3, tx : A2 });
  Serial2.on('data', function(d) { USB.write(d); });
  USB.on('data', function(d) { Serial2.write(d); });
  Serial1.setConsole();
  

• Disconnect from Espruino

• Now take 2 wires and connect one side of them to GND. Then connect to GPIO0, while connecting and releasing to RST as well (see here for where the pins are).

• finally, run this command using esptool and your firmware image: ./esptool.py -p /dev/ttyACM0 -b 9600 write_flash 0x00000 ../esptool_bin/AT25-SDK112-512k.bin

It's slooow at 9600 baud, but I haven't tried anything else yet. It does work though.