The mono feature can be activated either with a function button or via remote control (e.g. GPI contact closure). Any reason you don't want to activate via the function button?
If it is something you just want to activate and leave activated for extended periods of time you can set the function, activate the feature, then reassign the function button. This will keep it from being accidentally disabled. If on the other hand you want to be able to enable/disable the feature quickly then leaving that assigned to a function button would be your best option.