I've found a solution:

openerp.readOnlyCalendar = function(instance){
    var module = instance.web_calendar // loading the namespace of the 'web_calendar' module

    module.CalendarView.include({
        init_scheduler: function() {
            var readonly =this.fields_view.arch.attrs.ReadOnly
            if(readonly){
                scheduler.config.readonly= true;
            }
            this._super();  // calling the original CalendarView.init_scheduler() method
        }
    });
};

I use this code in a custom module: lp:~eirhylien/server-env-tools/readOnlyCalendar