How to approximate the behavior of a phototransistor in the circuit editor?
Try a current-controlled current source (CCCS) going to the base of the transistor. The current in the LED controls the current going to the base. Typical photo efficiency of an efficient device is about 1%, so the base current will be 0.01 times the LED current.

I don't know anything about CircuitLab, but I assume it has a CCCS.

Since this is a sensor, just simulate with different CCCS gains such as you have suggested.