Knockout: how to update another binding from custom binding?

We have usual problem with optionsCaption binding: it is always showed even when there is only one element. We solved this problem using our custom binding:

ko.bindingHandlers.optionsAutoSelect = {
    update: function (element, valueAccessor, allBindingsAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor());
        var allBindings = allBindingsAccessor();

        if (value.length == 1) {
            allBindings.optionsCaption = null;
        }
        ko.bindingHandlers.options.update(element, valueAccessor, allBindingsAccessor);
    }
};

after updating to knockout 3.0 allBindings become readonly. So any changes are really skipped. Any ideas how it could be solved in ko 3.0? We really have a lot of such auto selects and don't want to copy paste some computed code on all views. So we want some single option/extensibility point. Unfortunately as I could see options binding is rather monolithic.