Espruino Wifi as USB Master, Pico as USB slave - is that possible?

For the I2C slave, you could take a look at setWatch's data pin option (so you setWatch SCL and use SDA for data) - although things may still be a bit tricky to get it working nicely.

What I always suggest for things like this is just to use Serial communications with the UART. This generally works really nicely:

• Connect all the TX pins of slaves together, and all the RX pins
  
• Add a 10k pullup resistor to the Slave TX line
  
• Slave TX -> master RX, Slave RX -> master TX
  
• Ensure that after you set up all the Serial ports on the slaves, you set the pin mode of the TX pins to opendrain
  

• Create some kind of software system to ensure that the slaves only write data when they're asked to by the master. The simplest is just to use the built-in JS REPL to execute code that does some kind of if (id=myId) doSomething() type thing:

  // Slave
  var ID = 1; // change this for each slave
  
  // Master
  function sendCmd(id, cmd) {
  Serial1.println(`\x10if(id==${id})cmd;\n`);
  }
  function gotResponse(d) {
  print("Got data ",JSON.stringify(d));
  }
  sendCmd(1,"print('Hello')");
  var lineData = "";
  Serial1.on('data',function(l) {
  lineData+=l;
  var d = lineData.split("\n");
  lineData = d.pop();
  d.forEach(gotResponse);
  });