Not yet. It’s tricky because you need to use pin interrupts to get an accurate reading from a rotary encoder so it has to happen on the Arduino side. It may be possible to add a feature to Firmata to set pin interrupts, but the handler needs to be on the Arduino as well. There is some new work we are doing for Firmata that may make this possible. For now you would have to use custom firmware. See Breakout/custom_examples/simple_json.html and the accompanying sketch in Breakout/custom_examples/sketches/. The limitation with this approach however is that you cannot use both json and the normal Firmata features (analog I/O, Digitial I/O, I2C, etc) simultaneously. You’d have to pass all values using JSON strings.