I have been able to extend list view JS for custom header buttons and I can share that code...but very similar code won't work for kanban.  It doesn't matter how simple the JS is, as soon as I add anything that extends KanbanView the web.web_client fails to load.  I have tried this with every different event and some simple code that works with list...I'm working in Odoo 9.0 Community on this.

openerp.product_offer = function(instance, local) {	
    instance.web.KanbanView.include({		
            init: function(parent, dataset, view_id, options) {			
                this._super(parent, dataset, view_id, options);
            }	
    });
}