I have been using the url intercept method to pass data from javascript to objective C by passing the data as url encoded parameters and using NSURLProtocol to intercept the
Any request will be intercepted by the delegate so you can send any POST Ajax request, stuff it with the parameters and values that you want and then send it. All the values will be intercepted and you can use them the same way you are doing so far. An easy POST request could be sent using JQuery, as easy as:
$.post("toobjc.html", { 'data':"10k characters long string and more here ..." });
More here: http://api.jquery.com/jQuery.post/