This is good approach, basically called duck punching, see great explanation from Paul Irish
http://www.paulirish.com/2010/duck-punching-with-jquery/
I would only update the code to handle error also in case more arguments are passed into subscribe function, like this
var _subscribe = ko.subscribable.fn.subscribe;
ko.subscribable.fn.subscribe = function () {
try {
return _subscribe.apply(this, arguments);
}
catch (err) {
// handleError is a function in my code which will handle the error for me
handleError(err);
}
};