This one is easy, as we are already doing a check in the watcher to find out whether the selectedValue is undefined or not. As such, we can extend the conditional check to include whether the read-only property is true or false:

scope.$watch('selectedValue', function() {
    if(typeof scope.selectedValue !== "undefined"                             && !scope.layout.readonly){
         app.field(scope.layout.field)
            .select([parseInt(scope.selectedValue,10)])
 }
});