I assume the monobrick communication library will work on the monobrick firmware – so should not be the problem?
No that is not the case. The MonoBrick firmware has no implementation for the LEGO direct or system protocol.
With the MonoBrick EV3 firmware you have a lot of other possibilities for remote controlling your EV3. The most obvious is to use a socket connection. You can use this example to get started.
Another option is to look at the webserver example found here. This will let you control the brick from a browser. This guy wrote his own
webser that lets him control the robot from any phone or PC/Mac via a browser. Hope this answers your question.